cc: Cleanup DelayBasedTimeSource code.

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 9eb3ea279daf7c6d04b656d06c25b87d65a94b50..82acfbead1a9364259d8f1dee8101d3d59e4d5d5 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -225,6 +225,7 @@ class CC_EXPORT SyntheticBeginFrameSource : public BeginFrameSourceBase,
 
  protected:
   explicit SyntheticBeginFrameSource(
+      base::SingleThreadTaskRunner* task_runner,
       scoped_ptr<DelayBasedTimeSource> time_source);
 
   BeginFrameArgs CreateBeginFrameArgs(base::TimeTicks frame_time,
@@ -233,9 +234,12 @@ class CC_EXPORT SyntheticBeginFrameSource : public BeginFrameSourceBase,
   // BeginFrameSourceBase
   void OnNeedsBeginFramesChange(bool needs_begin_frames) override;
 
+  base::SingleThreadTaskRunner* task_runner_;
   scoped_ptr<DelayBasedTimeSource> time_source_;
 
  private:
+  base::WeakPtrFactory<SyntheticBeginFrameSource> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(SyntheticBeginFrameSource);
 };