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diff --git a/xpcom/threads/IdlePeriodState.h b/xpcom/threads/IdlePeriodState.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_IdlePeriodState_h
+#define mozilla_IdlePeriodState_h
+
+/**
+ * A class for tracking the state of our idle period.  This includes keeping
+ * track of both the state of our process-local idle period estimate and, for
+ * content processes, managing communication with the parent process for
+ * cross-pprocess idle detection.
+ */
+
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/RefPtr.h"
+#include "mozilla/TimeStamp.h"
+#include "nsCOMPtr.h"
+
+#include <stdint.h>
+
+class nsIIdlePeriod;
+
+namespace mozilla {
+class TaskManager;
+namespace ipc {
+class IdleSchedulerChild;
+}  // namespace ipc
+
+class IdlePeriodState {
+ public:
+  explicit IdlePeriodState(already_AddRefed<nsIIdlePeriod>&& aIdlePeriod);
+
+  ~IdlePeriodState();
+
+  // Integration with memory reporting.
+  size_t SizeOfExcludingThis(MallocSizeOf aMallocSizeOf) const;
+
+  // Notification that whoever we are tracking idle state for has found a
+  // non-idle task to process.
+  //
+  // Must not be called while holding any locks.
+  void FlagNotIdle();
+
+  // Notification that whoever we are tracking idle state for has no more
+  // tasks (idle or not) to process.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  void RanOutOfTasks(const MutexAutoUnlock& aProofOfUnlock);
+
+  // Notification that whoever we are tracking idle state has idle tasks that
+  // they are considering ready to run and that we should keep claiming they are
+  // ready to run until they call ForgetPendingTaskGuarantee().
+  void EnforcePendingTaskGuarantee() {
+    mHasPendingEventsPromisedIdleEvent = true;
+  }
+
+  // Notification that whoever we are tracking idle state for is done with our
+  // "we have an idle event ready to run" guarantee.  When this happens, we can
+  // reset mHasPendingEventsPromisedIdleEvent to false, because we have
+  // fulfilled our contract.
+  void ForgetPendingTaskGuarantee() {
+    mHasPendingEventsPromisedIdleEvent = false;
+  }
+
+  // Update our cached idle deadline so consumers can use it while holding
+  // locks. Consumers must ClearCachedIdleDeadline() once they are done.
+  void UpdateCachedIdleDeadline(const MutexAutoUnlock& aProofOfUnlock) {
+    mCachedIdleDeadline = GetIdleDeadlineInternal(false, aProofOfUnlock);
+  }
+
+  // If we have local idle deadline, but don't have an idle token, this will
+  // request such from the parent process when this is called in a child
+  // process.
+  void RequestIdleDeadlineIfNeeded(const MutexAutoUnlock& aProofOfUnlock) {
+    GetIdleDeadlineInternal(false, aProofOfUnlock);
+  }
+
+  // Reset our cached idle deadline, so we stop allowing idle runnables to run.
+  void ClearCachedIdleDeadline() { mCachedIdleDeadline = TimeStamp(); }
+
+  // Get the current cached idle deadline.  This may return a null timestamp.
+  TimeStamp GetCachedIdleDeadline() { return mCachedIdleDeadline; }
+
+  // Peek our current idle deadline into mCachedIdleDeadline.  This can cause
+  // mCachedIdleDeadline to be a null timestamp (which means we are not idle
+  // right now).  This method does not have any side-effects on our state, apart
+  // from guaranteeing that if it returns non-null then GetDeadlineForIdleTask
+  // will return non-null until ForgetPendingTaskGuarantee() is called, and its
+  // effects on mCachedIdleDeadline.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  void CachePeekedIdleDeadline(const MutexAutoUnlock& aProofOfUnlock) {
+    mCachedIdleDeadline = GetIdleDeadlineInternal(true, aProofOfUnlock);
+  }
+
+  void SetIdleToken(uint64_t aId, TimeDuration aDuration);
+
+  bool IsActive() { return mActive; }
+
+ protected:
+  void EnsureIsActive() {
+    if (!mActive) {
+      SetActive();
+    }
+  }
+
+  void EnsureIsPaused(const MutexAutoUnlock& aProofOfUnlock) {
+    if (mActive) {
+      SetPaused(aProofOfUnlock);
+    }
+  }
+
+  // Returns a null TimeStamp if we're not in the idle period.
+  TimeStamp GetLocalIdleDeadline(bool& aShuttingDown,
+                                 const MutexAutoUnlock& aProofOfUnlock);
+
+  // Gets the idle token, which is the end time of the idle period.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  TimeStamp GetIdleToken(TimeStamp aLocalIdlePeriodHint,
+                         const MutexAutoUnlock& aProofOfUnlock);
+
+  // In case of child processes, requests idle time from the cross-process
+  // idle scheduler.
+  void RequestIdleToken(TimeStamp aLocalIdlePeriodHint);
+
+  // Mark that we don't have idle time to use, nor are expecting to get an idle
+  // token from the idle scheduler.  This must be called while not holding any
+  // locks, but some of the callers aren't holding locks to start with, so
+  // consumers just need to make sure they are not holding locks when they call
+  // this.
+  void ClearIdleToken();
+
+  // SetActive should be called when the event queue is running any type of
+  // tasks.
+  void SetActive();
+  // SetPaused should be called once the event queue doesn't have more
+  // tasks to process, or is waiting for the idle token.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  void SetPaused(const MutexAutoUnlock& aProofOfUnlock);
+
+  // Get or peek our idle deadline.  When peeking, we generally don't change any
+  // of our internal state.  When getting, we may request an idle token as
+  // needed.
+  //
+  // aProofOfUnlock is the proof that our caller unlocked its mutex.
+  TimeStamp GetIdleDeadlineInternal(bool aIsPeek,
+                                    const MutexAutoUnlock& aProofOfUnlock);
+
+  // Whether we should be getting an idle token (i.e. are a content process
+  // and are using cross process idle scheduling).
+  bool ShouldGetIdleToken();
+
+  // Set to true if we have claimed we have a ready-to-run idle task when asked.
+  // In that case, we will ensure that we allow at least one task to run when
+  // someone tries to run a task, even if we have run out of idle period at that
+  // point.  This ensures that we never fail to produce a task to run if we
+  // claim we have a task ready to run.
+  bool mHasPendingEventsPromisedIdleEvent = false;
+
+  // mIdlePeriod keeps track of the current idle period. Calling
+  // mIdlePeriod->GetIdlePeriodHint() will give an estimate of when
+  // the current idle period will end.
+  nsCOMPtr<nsIIdlePeriod> mIdlePeriod;
+
+  // If non-null, this timestamp represents the end time of the idle period.  An
+  // idle period starts when we get the idle token from the parent process and
+  // ends when either there are no more things we want to run at idle priority
+  // or mIdleToken < TimeStamp::Now(), so we have reached our idle deadline.
+  TimeStamp mIdleToken;
+
+  // The id of the last idle request to the cross-process idle scheduler.
+  uint64_t mIdleRequestId = 0;
+
+  // If we're in a content process, we use mIdleScheduler to communicate with
+  // the parent process for purposes of cross-process idle tracking.
+  RefPtr<mozilla::ipc::IdleSchedulerChild> mIdleScheduler;
+
+  // Our cached idle deadline.  This is set by UpdateCachedIdleDeadline() and
+  // cleared by ClearCachedIdleDeadline().  Consumers should do the former while
+  // not holding any locks, but may do the latter while holding locks.
+  TimeStamp mCachedIdleDeadline;
+
+  // mActive is true when the PrioritizedEventQueue or TaskController we are
+  // associated with is running tasks.
+  bool mActive = true;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_IdlePeriodState_h