Route D3D VSync signal to Compositor

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 58ee480d01138cc235f49ec13456019bb97b23f1..2eb177c5e2db9b3a489b813bc7e624a83e422efd 100644
--- a/cc/scheduler/begin_frame_source.h
+++ b/cc/scheduler/begin_frame_source.h
@@ -147,11 +147,14 @@ class CC_EXPORT StubBeginFrameSource : public BeginFrameSource {
   bool IsThrottled() const override;
 };
 
-// A frame source which ticks itself independently.
+// A frame source which parameters are updated externally.
 class CC_EXPORT SyntheticBeginFrameSource : public BeginFrameSource {
  public:
   ~SyntheticBeginFrameSource() override;
 
+  // Depending on implementation this might either update parameters used to
+  // generate BeginFrame signal in BeginFrameSource or be used as an actual
+  // VSync signal.
   virtual void OnUpdateVSyncParameters(base::TimeTicks timebase,
                                        base::TimeDelta interval) = 0;
   // This overrides any past or future interval from updating vsync parameters.