cc: Make separate interface for BeginFrame ipc from OutputSurface

cc/test/layer_tree_test.h

Index: cc/test/layer_tree_test.h
diff --git a/cc/test/layer_tree_test.h b/cc/test/layer_tree_test.h
index 0eb4dd2ddf0b6ad2b43b6e71209aeba10e4a391a..2fdaa9392f24b9d10c8137c81a29d257458e73e6 100644
--- a/cc/test/layer_tree_test.h
+++ b/cc/test/layer_tree_test.h
@@ -17,6 +17,7 @@ class WebGraphicsContext3D;
 }
 
 namespace cc {
+class ExternalBeginFrameSourceForTest;
 class FakeLayerTreeHostClient;
 class FakeOutputSurface;
 class LayerImpl;
@@ -208,6 +209,7 @@ class LayerTreeTest : public testing::Test, public TestHooks {
   scoped_ptr<LayerTreeHostClientForTesting> client_;
   scoped_ptr<LayerTreeHost> layer_tree_host_;
   FakeOutputSurface* output_surface_;
+  ExternalBeginFrameSourceForTest* external_begin_frame_source_;
 
   bool beginning_;
   bool end_when_begin_returns_;