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 1e1334dfd28af283638bbaa8af466b8416bf87c7..7e43aed6296592fe1f20e024c30f328f3f8d601e 100644
--- a/cc/layer_tree_host_unittest.cc
+++ b/cc/layer_tree_host_unittest.cc
@@ -686,7 +686,7 @@ 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;
         }
     }