diff -r 000000000000 -r 6474c204b198 xpcom/threads/nsMemoryPressure.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/threads/nsMemoryPressure.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,53 @@
+/* -*- 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/. */
+
+#include "nsMemoryPressure.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+
+#include "nsThreadUtils.h"
+
+using namespace mozilla;
+
+static Atomic<int32_t, Relaxed> sMemoryPressurePending;
+static_assert(MemPressure_None == 0,
+              "Bad static initialization with the default constructor.");
+
+MemoryPressureState
+NS_GetPendingMemoryPressure()
+{
+  int32_t value = sMemoryPressurePending.exchange(MemPressure_None);
+  return MemoryPressureState(value);
+}
+
+void
+NS_DispatchEventualMemoryPressure(MemoryPressureState state)
+{
+  /*
+   * A new memory pressure event erases an ongoing memory pressure, but an
+   * existing "new" memory pressure event takes precedence over a new "ongoing"
+   * memory pressure event.
+   */
+  switch (state) {
+    case MemPressure_None:
+      sMemoryPressurePending = MemPressure_None;
+      break;
+    case MemPressure_New:
+      sMemoryPressurePending = MemPressure_New;
+      break;
+    case MemPressure_Ongoing:
+      sMemoryPressurePending.compareExchange(MemPressure_None, MemPressure_Ongoing);
+      break;
+  }
+}
+
+nsresult
+NS_DispatchMemoryPressure(MemoryPressureState state)
+{
+  NS_DispatchEventualMemoryPressure(state);
+  nsCOMPtr<nsIRunnable> event = new nsRunnable;
+  return NS_DispatchToMainThread(event);
+}