Reimplement vsync latency UMA to be based on BeginImplFrame rather than BeginMainFrame

cc/scheduler/scheduler.h

Index: cc/scheduler/scheduler.h
diff --git a/cc/scheduler/scheduler.h b/cc/scheduler/scheduler.h
index 30dd6c8144e2a681953d9805fbab5dd2a4d3d0bc..15c45260704bc1cbbe55261437a9d4cb2d542885 100644
--- a/cc/scheduler/scheduler.h
+++ b/cc/scheduler/scheduler.h
@@ -206,7 +206,7 @@ class CC_EXPORT Scheduler : public BeginFrameObserverBase {
   bool IsBeginMainFrameSentOrStarted() const;
   void BeginImplFrameWithDeadline(const BeginFrameArgs& args);
   void BeginImplFrameSynchronous(const BeginFrameArgs& args);
-  void BeginImplFrame(const BeginFrameArgs& args);
+  void BeginImplFrame(const BeginFrameArgs& args, base::TimeTicks now);
   void FinishImplFrame();
   enum BeginFrameResult { kBeginFrameSkipped, kBeginFrameFinished };
   void SendBeginFrameAck(const BeginFrameArgs& args, BeginFrameResult result);