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diff --git a/dom/media/doctor/DDLifetime.h b/dom/media/doctor/DDLifetime.h
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+++ b/dom/media/doctor/DDLifetime.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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 DDLifetime_h_
+#define DDLifetime_h_
+
+#include "DDLogObject.h"
+#include "DDMessageIndex.h"
+#include "DDTimeStamp.h"
+
+namespace mozilla {
+
+namespace dom {
+class HTMLMediaElement;
+}  // namespace dom
+
+// This struct records the lifetime of one C++ object.
+// Note that multiple objects may have the same address and type (at different
+// times), so the recorded construction/destruction times should be used to
+// distinguish them.
+struct DDLifetime {
+  const DDLogObject mObject;
+  const DDMessageIndex mConstructionIndex;
+  const DDTimeStamp mConstructionTimeStamp;
+  // Only valid when mDestructionTimeStamp is not null.
+  DDMessageIndex mDestructionIndex;
+  DDTimeStamp mDestructionTimeStamp;
+  // Associated HTMLMediaElement, initially nullptr until this object can be
+  // linked to its HTMLMediaElement.
+  const dom::HTMLMediaElement* mMediaElement;
+  // If not null, derived object for which this DDLifetime is a base class.
+  // This is used to link messages from the same object, even when they
+  // originate from a method on a base class.
+  // Note: We assume a single-inheritance hierarchy.
+  DDLogObject mDerivedObject;
+  DDMessageIndex mDerivedObjectLinkingIndex;
+  // Unique tag used to identify objects in a log, easier to read than object
+  // pointers.
+  // Negative and unique for unassociated objects.
+  // Positive for associated objects, and unique for that HTMLMediaElement
+  // group.
+  int32_t mTag;
+
+  DDLifetime(DDLogObject aObject, DDMessageIndex aConstructionIndex,
+             DDTimeStamp aConstructionTimeStamp, int32_t aTag)
+      : mObject(aObject),
+        mConstructionIndex(aConstructionIndex),
+        mConstructionTimeStamp(aConstructionTimeStamp),
+        mDestructionIndex(0),
+        mMediaElement(nullptr),
+        mDerivedObjectLinkingIndex(0),
+        mTag(aTag) {}
+
+  // Is this lifetime alive at the given index?
+  // I.e.: Constructed before, and destroyed later or not yet.
+  bool IsAliveAt(DDMessageIndex aIndex) const {
+    return aIndex >= mConstructionIndex &&
+           (!mDestructionTimeStamp || aIndex <= mDestructionIndex);
+  }
+
+  // Print the object's pointer, tag and class name (and derived class). E.g.:
+  // "dom::HTMLVideoElement[134073800]#1 (as dom::HTMLMediaElement)"
+  void AppendPrintf(nsCString& aString) const;
+  nsCString Printf() const;
+};
+
+}  // namespace mozilla
+
+#endif  // DDLifetime_h_