cc: Plumbing for BeginFrameSource based on Surfaces

cc/surfaces/display_scheduler.h

Index: cc/surfaces/display_scheduler.h
diff --git a/cc/surfaces/display_scheduler.h b/cc/surfaces/display_scheduler.h
index 4ffc6ea8d56440abcf81fd9058556786b7f3aa9c..7b043c04cd26ac3c753cfd85a33dd3180d9402de 100644
--- a/cc/surfaces/display_scheduler.h
+++ b/cc/surfaces/display_scheduler.h
@@ -48,6 +48,10 @@ class CC_SURFACES_EXPORT DisplayScheduler : public BeginFrameObserverBase {
   // BeginFrameObserverBase implementation
   bool OnBeginFrameDerivedImpl(const BeginFrameArgs& args) override;
 
+  BeginFrameSource* begin_frame_source_for_children() {
+    return begin_frame_source_for_children_.get();
+  }
+
  protected:
   base::TimeTicks DesiredBeginFrameDeadlineTime();
   virtual void ScheduleBeginFrameDeadline();
@@ -64,6 +68,9 @@ class CC_SURFACES_EXPORT DisplayScheduler : public BeginFrameObserverBase {
   base::CancelableClosure begin_frame_deadline_task_;
   base::TimeTicks begin_frame_deadline_task_time_;
 
+  // TODO(tansell): Set this to something useful.
+  scoped_ptr<BeginFrameSource> begin_frame_source_for_children_;
+
   bool output_surface_lost_;
   bool root_surface_resources_locked_;