Bug 1419363 - Switch to WeakPtr. r=jib

--HG--
extra : rebase_source : 7148328b61e25c3eac983e41f1ece633bf2206bf

dom/html/HTMLMediaElement.cpp

@@ -5237,11 +5237,14 @@ public:
   {
     NS_ASSERTION(NS_IsMainThread(), "Should be on main thread.");
 
+    if (!mElement) {
+      return;
+    }
     mElement->NotifyMediaStreamTracksAvailable(aStream);
   }
 
 private:
-  HTMLMediaElement* mElement;
+  WeakPtr<HTMLMediaElement> mElement;
 };
 
 class HTMLMediaElement::MediaStreamTrackListener :