Allow cc::Scheduler begin frame source to be changed

cc/scheduler/scheduler.h

Index: cc/scheduler/scheduler.h
diff --git a/cc/scheduler/scheduler.h b/cc/scheduler/scheduler.h
index 4828896194813409e3d607b8d07d1078ea4a5c52..21c9d04b661415ecda67aeb56d1d20b614b2f68a 100644
--- a/cc/scheduler/scheduler.h
+++ b/cc/scheduler/scheduler.h
@@ -80,6 +80,7 @@ class CC_EXPORT Scheduler : public BeginFrameObserverBase {
   void SetCanDraw(bool can_draw);
   void NotifyReadyToActivate();
   void NotifyReadyToDraw();
+  void SetBeginFrameSource(BeginFrameSource* source);
 
   void SetNeedsBeginMainFrame();
   // Requests a single impl frame (after the current frame if there is one
@@ -140,6 +141,10 @@ class CC_EXPORT Scheduler : public BeginFrameObserverBase {
   void SetChildrenNeedBeginFrames(bool children_need_begin_frames);
   void SetVideoNeedsBeginFrames(bool video_needs_begin_frames);
 
+  const BeginFrameSource* begin_frame_source() const {
+    return begin_frame_source_;
+  }
+
  protected:
   Scheduler(SchedulerClient* client,
             const SchedulerSettings& scheduler_settings,