cc: Make separate interface for BeginFrame ipc from OutputSurface

cc/scheduler/begin_frame_source.h

Index: cc/scheduler/begin_frame_source.h
diff --git a/cc/scheduler/begin_frame_source.h b/cc/scheduler/begin_frame_source.h
index 3d849a40d638708ad37acca859aaeaa98e9e911e..e96a3e7fa558745e40fff78921f805108e06183c 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -117,6 +117,9 @@ class CC_EXPORT BeginFrameSource {
   virtual void AddObserver(BeginFrameObserver* obs) = 0;
   virtual void RemoveObserver(BeginFrameObserver* obs) = 0;
 
+  // Tells the Source that client is ready to handle BeginFrames messages.
+  virtual void SetClientReady() = 0;
+
   // Tracing support - Recommend (but not required) to call this implementation
   // in any override.
   virtual void AsValueInto(base::debug::TracedValue* dict) const = 0;
@@ -139,6 +142,7 @@ class CC_EXPORT BeginFrameSourceMixIn : public BeginFrameSource {
   void DidFinishFrame(size_t remaining_frames) override {}
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
+  void SetClientReady() override {}
 
   // Tracing support - Recommend (but not required) to call this implementation
   // in any override.