cc: Making BeginFrameSources support multiple observers.

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 f6f1a789da7bb5770d8e4c5ae960de1b60ab14d5..fe6c5101ce35506af96b42d98d7b103cbf7b3a3b 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -136,7 +136,7 @@ class CC_EXPORT BeginFrameSource {
 //    in their own AsValueInto implementation.
 class CC_EXPORT BeginFrameSourceBase : public BeginFrameSource {
  public:
-  ~BeginFrameSourceBase() override {}
+  ~BeginFrameSourceBase() override;
 
   // BeginFrameSource
   bool NeedsBeginFrames() const final;
@@ -161,7 +161,9 @@ class CC_EXPORT BeginFrameSourceBase : public BeginFrameSource {
   // needs_begin_frames change.
   virtual void OnNeedsBeginFramesChange(bool needs_begin_frames) {}
 
-  BeginFrameObserver* observer_;
+  base::ObserverList<const BeginFrameObserver>::Iterator
+  getConstObserverListIterator() const;
+  base::ObserverList<BeginFrameObserver> observer_list_;
   bool needs_begin_frames_;
 
  private: