Bug 1560931 - Fire memory-pressure-stop events on Windows r=gsvelto

I'd like to rely on always getting a stop event when the system recovers from a memory pressure situation.  AIUI this already happens on Android platforms but not on Windows.  This adds the stop event for Windows.

Differential Revision: https://phabricator.services.mozilla.com/D35679

xpcom/base/AvailableMemoryTracker.cpp

@@ -94,6 +94,7 @@ class nsAvailableMemoryWatcher final : public nsIObserver,
   bool OngoingMemoryPressure() { return mUnderMemoryPressure; }
   void AdjustPollingInterval(const bool aLowMemory);
   void SendMemoryPressureEvent();
+  void MaybeSendMemoryPressureStopEvent();
   void MaybeSaveMemoryReport();
   void Shutdown();
 
@@ -188,6 +189,12 @@ void nsAvailableMemoryWatcher::SendMemoryPressureEvent() {
   NS_DispatchEventualMemoryPressure(state);
 }
 
+void nsAvailableMemoryWatcher::MaybeSendMemoryPressureStopEvent() {
+  if (OngoingMemoryPressure()) {
+    NS_DispatchEventualMemoryPressure(MemPressure_Stopping);
+  }
+}
+
 void nsAvailableMemoryWatcher::MaybeSaveMemoryReport() {
   if (!mSavedReport && OngoingMemoryPressure()) {
     nsCOMPtr<nsICrashReporter> cr =
@@ -221,6 +228,11 @@ nsAvailableMemoryWatcher::Notify(nsITimer* aTimer) {
 
   if (lowMemory) {
     SendMemoryPressureEvent();
+  } else {
+    MaybeSendMemoryPressureStopEvent();
+  }
+
+  if (lowMemory) {
     MaybeSaveMemoryReport();
   } else {
     mSavedReport = false;  // Save a new report if memory gets low again

xpcom/threads/nsMemoryPressure.h

@@ -55,7 +55,10 @@ enum MemoryPressureState {
 
 /**
  * Return and erase the latest state of the memory pressure event set by any of
- * the corresponding dispatch function.
+ * the corresponding dispatch functions.
+ *
+ * This is called when processing events on the main thread to check whether to
+ * fire a memory pressure notification.
  */
 MemoryPressureState NS_GetPendingMemoryPressure();