cc: Use correct message loop proxy in BlockingTaskRunner

cc/trees/proxy.h

Index: cc/trees/proxy.h
diff --git a/cc/trees/proxy.h b/cc/trees/proxy.h
index 112c259dc47c04bb6eb3f45f9c517390f885c741..7dd5e6e516760f673f3cd077328ec98cead6b2c1 100644
--- a/cc/trees/proxy.h
+++ b/cc/trees/proxy.h
@@ -30,6 +30,7 @@ class Vector2d;
 
 namespace cc {
 
+class BlockingTaskRunner;
 class LayerTreeDebugState;
 class OutputSurface;
 struct RendererCapabilities;
@@ -113,6 +114,11 @@ class CC_EXPORT Proxy {
  private:
   scoped_refptr<base::SingleThreadTaskRunner> main_task_runner_;
   scoped_refptr<base::SingleThreadTaskRunner> impl_task_runner_;
+
+  // Blocking task runner which forwards tasks to |main_task_runner_| while the
+  // Proxy is alive.
+  scoped_refptr<BlockingTaskRunner> blocking_main_thread_task_runner_;
+
 #if DCHECK_IS_ON
   const base::PlatformThreadId main_thread_id_;
   bool impl_thread_is_overridden_;