cc:: Change plumbing for external_begin_frame_source to the Scheduler.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 3bfd73ca5c9cd4108ee22462b1299c98330b5333..f7d3e966e9ec63c6426c1fcb04c4df706dba7ddf 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -369,8 +369,10 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
       LayerTreeHostSingleThreadClient* single_thread_client,
       scoped_refptr<base::SingleThreadTaskRunner> main_task_runner,
       scoped_ptr<BeginFrameSource> external_begin_frame_source);
-  void InitializeForTesting(scoped_ptr<TaskRunnerProvider> task_runner_provider,
-                            scoped_ptr<Proxy> proxy_for_testing);
+  void InitializeForTesting(
+      scoped_ptr<TaskRunnerProvider> task_runner_provider,
+      scoped_ptr<Proxy> proxy_for_testing,
+      scoped_ptr<BeginFrameSource> external_begin_frame_source);
   void SetOutputSurfaceLostForTesting(bool is_lost) {
     output_surface_lost_ = is_lost;
   }
@@ -391,7 +393,9 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
   void OnCommitForSwapPromises();
 
  private:
-  void InitializeProxy(scoped_ptr<Proxy> proxy);
+  void InitializeProxy(
+      scoped_ptr<Proxy> proxy,
+      scoped_ptr<BeginFrameSource> external_begin_frame_source);
 
   bool DoUpdateLayers(Layer* root_layer);
   void UpdateHudLayer();