Use WeakPtr for posting cc tasks to main thread

cc/layer_tree_host_unittest.cc

Index: cc/layer_tree_host_unittest.cc
diff --git a/cc/layer_tree_host_unittest.cc b/cc/layer_tree_host_unittest.cc
index 887454bd46cb411ed1a811e7b9ecd24436219115..486c44249118ec0f8c492903ad52a1ac33f70cab 100644
--- a/cc/layer_tree_host_unittest.cc
+++ b/cc/layer_tree_host_unittest.cc
@@ -797,7 +797,7 @@ public:
 
         // We request animation only once.
         if (!m_animationRequested) {
-            m_mainThreadProxy->postTask(FROM_HERE, base::Bind(&LayerTreeHostTestStartPageScaleAnimation::requestStartPageScaleAnimation, base::Unretained(this)));
+            impl->proxy()->mainThread()->postTask(base::Bind(&LayerTreeHostTestStartPageScaleAnimation::requestStartPageScaleAnimation, base::Unretained(this)));
             m_animationRequested = true;
         }
     }