Bug 1208371 - Guard LoadManagerSingleton with a WeakPtr. r=pkerr

LoadManagerSingleton has a separate shutdown path (xpcom-shutdown) from
its users (Audio/VideoConduit - garbage collected). These have appeared
racy, so in some cases the singleton was destructed before the users had
deregistered (e.g., when conduits destructed by SnowWhiteKiller).

A WeakPtr can solve this.

MozReview-Commit-ID: AVrpd3QqOGx

--HG--
extra : rebase_source : c5d50f7619940b772c7d1f3dee3ac0b6568ccfd9

dom/media/systemservices/LoadManager.h

@@ -7,6 +7,7 @@
 #define _LOADMANAGER_H_
 
 #include "LoadMonitor.h"
+#include "mozilla/WeakPtr.h"
 #include "mozilla/StaticPtr.h"
 #include "mozilla/TimeStamp.h"
 #include "mozilla/Services.h"
@@ -23,12 +24,15 @@ namespace mozilla {
 class LoadManagerSingleton : public LoadNotificationCallback,
                              public webrtc::CPULoadStateCallbackInvoker,
                              public webrtc::CpuOveruseObserver,
+                             public SupportsWeakPtr<LoadManagerSingleton>,
                              public nsIObserver
 
 {
 public:
     static LoadManagerSingleton* Get();
 
+    MOZ_DECLARE_WEAKREFERENCE_TYPENAME(LoadManagerSingleton)
+
     NS_DECL_THREADSAFE_ISUPPORTS
     NS_DECL_NSIOBSERVER
 
@@ -86,23 +90,31 @@ public:
 
     void AddObserver(webrtc::CPULoadStateObserver * aObserver) override
     {
+        if (mManager) {
           mManager->AddObserver(aObserver);
         }
+    }
     void RemoveObserver(webrtc::CPULoadStateObserver * aObserver) override
     {
+        if (mManager) {
           mManager->RemoveObserver(aObserver);
         }
+    }
     void OveruseDetected() override
     {
+        if (mManager) {
           mManager->OveruseDetected();
         }
+    }
     void NormalUsage() override
     {
+        if (mManager) {
           mManager->NormalUsage();
         }
+    }
 
 private:
-    LoadManagerSingleton* mManager;
+    WeakPtr<LoadManagerSingleton> mManager;
 };
 
 } //namespace