[wip] hack to use synthetic bfs for browser compositor

cc/trees/single_thread_proxy.h

Index: cc/trees/single_thread_proxy.h
diff --git a/cc/trees/single_thread_proxy.h b/cc/trees/single_thread_proxy.h
index 2fdcfd60798c27deae8c6cdcf6124e0e5a7bc580..f8e9f27ddb03c240869eceac89834acc7dc2039e 100644
--- a/cc/trees/single_thread_proxy.h
+++ b/cc/trees/single_thread_proxy.h
@@ -129,6 +129,7 @@ class CC_EXPORT SingleThreadProxy : public Proxy,
   // Used on the Thread, but checked on main thread during
   // initialization/shutdown.
   std::unique_ptr<LayerTreeHostImpl> layer_tree_host_impl_;
+  std::unique_ptr<SyntheticBeginFrameSource> synthetic_begin_frame_source_;
 
   // Accessed from both threads.
   std::unique_ptr<Scheduler> scheduler_on_impl_thread_;