Bug 790854. Set mMainThreadDestroyed before the stream is destroyed. r=jesup

--HG--
extra : rebase_source : 6b1d2a64b9af082b02a207759f051cb6d7975e35

content/media/MediaStreamGraph.cpp

@@ -1798,8 +1798,9 @@ MediaStream::Destroy()
     { Run(); }
   };
   mWrapper = nullptr;
-  GraphImpl()->AppendMessage(new Message(this));
   mMainThreadDestroyed = true;
+  GraphImpl()->AppendMessage(new Message(this));
+  // 'this' can be dead after the AppendMessage, so don't touch it here.
 }
 
 void