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diff --git a/xpcom/base/MemoryPressureLevelMac.h b/xpcom/base/MemoryPressureLevelMac.h
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+++ b/xpcom/base/MemoryPressureLevelMac.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_MemoryPressureLevelMac_h
+#define mozilla_MemoryPressureLevelMac_h
+
+namespace mozilla {
+
+#if defined(XP_DARWIN)
+// An internal representation of the Mac memory-pressure level constants.
+class MacMemoryPressureLevel {
+ public:
+  // Order enum values so that higher integer values represent higher
+  // memory pressure levels allowing comparison operators to be used.
+  enum class Value {
+    eUnset,
+    eUnexpected,
+    eNormal,
+    eWarning,
+    eCritical,
+  };
+
+  MacMemoryPressureLevel() : mValue(Value::eUnset) {}
+  MOZ_IMPLICIT MacMemoryPressureLevel(Value aValue) : mValue(aValue) {}
+
+  bool operator==(const Value& aRhsValue) const { return mValue == aRhsValue; }
+  bool operator==(const MacMemoryPressureLevel& aRhs) const {
+    return mValue == aRhs.mValue;
+  }
+
+  // Implement '<' and derive the other comparators from it.
+  bool operator<(const MacMemoryPressureLevel& aRhs) const {
+    return mValue < aRhs.mValue;
+  }
+  bool operator>(const MacMemoryPressureLevel& aRhs) const {
+    return aRhs < *this;
+  }
+  bool operator<=(const MacMemoryPressureLevel& aRhs) const {
+    return !(aRhs < *this);
+  }
+  bool operator>=(const MacMemoryPressureLevel& aRhs) const {
+    return !(*this < aRhs);
+  }
+
+  Value GetValue() { return mValue; }
+  bool IsNormal() { return mValue == Value::eNormal; }
+  bool IsUnsetOrNormal() { return IsNormal() || (mValue == Value::eUnset); }
+  bool IsWarningOrAbove() {
+    return (mValue == Value::eWarning) || (mValue == Value::eCritical);
+  }
+
+  const char* ToString() {
+    switch (mValue) {
+      case Value::eUnset:
+        return "Unset";
+      case Value::eUnexpected:
+        return "Unexpected";
+      case Value::eNormal:
+        return "Normal";
+      case Value::eWarning:
+        return "Warning";
+      case Value::eCritical:
+        return "Critical";
+    }
+  }
+
+ private:
+  Value mValue;
+};
+#endif
+
+}  // namespace mozilla
+
+#endif  // mozilla_MemoryPressureLevelMac_h