Make MDSM::StartBuffering happen on the state machine thread

dom/media/MediaDecoderStateMachine.h

@@ -254,11 +254,17 @@ public:
   // the decode monitor held.
   void UpdatePlaybackPosition(int64_t aTime);
 
+private:
   // Causes the state machine to switch to buffering state, and to
-  // immediately stop playback and buffer downloaded data. Must be called
-  // with the decode monitor held. Called on the state machine thread and
-  // the main thread.
+  // immediately stop playback and buffer downloaded data. Called on
+  // the state machine thread.
   void StartBuffering();
+public:
+
+  void DispatchStartBuffering()
+  {
+    TaskQueue()->Dispatch(NS_NewRunnableMethod(this, &MediaDecoderStateMachine::StartBuffering));
+  }
 
   // This is called on the state machine thread and audio thread.
   // The decoder monitor must be obtained before calling this.