Use WeakPtr for posting cc tasks to main thread

cc/thread_proxy.h

Index: cc/thread_proxy.h
diff --git a/cc/thread_proxy.h b/cc/thread_proxy.h
index f661a363a15c0a644e220df7e230d433515e16c4..bce19dc8d2630dd632d879a015b1c53dd39cf321 100644
--- a/cc/thread_proxy.h
+++ b/cc/thread_proxy.h
@@ -152,14 +152,15 @@ private:
 
     base::WeakPtrFactory<ThreadProxy> m_weakFactoryOnImplThread;
 
+    base::WeakPtr<ThreadProxy> m_mainThreadWeakPtr;
+    base::WeakPtrFactory<ThreadProxy> m_weakFactory;
+
     scoped_ptr<LayerTreeHostImpl> m_layerTreeHostImpl;
 
     scoped_ptr<InputHandler> m_inputHandlerOnImplThread;
 
     scoped_ptr<Scheduler> m_schedulerOnImplThread;
 
-    scoped_refptr<ScopedThreadProxy> m_mainThreadProxy;
-
     // Holds on to the context we might use for compositing in between initializeContext()
     // and initializeRenderer() calls.
     scoped_ptr<OutputSurface> m_outputSurfaceBeforeInitializationOnImplThread;