cc: Measure compositor timing with finer granularity

cc/trees/thread_proxy.h

Index: cc/trees/thread_proxy.h
diff --git a/cc/trees/thread_proxy.h b/cc/trees/thread_proxy.h
index 672fca50ded05671e2027c1d6be19635304af519..ca76e42669b1cbfbd1f3e19dda3a2bbced27aa7a 100644
--- a/cc/trees/thread_proxy.h
+++ b/cc/trees/thread_proxy.h
@@ -197,6 +197,7 @@ class CC_EXPORT ThreadProxy : public Proxy,
   void PostDelayedAnimationTaskOnImplThread(const base::Closure& task,
                                             base::TimeDelta delay) override;
   void DidActivateSyncTree() override;
+  void WillPrepareTiles() override;
   void DidPrepareTiles() override;
   void DidCompletePageScaleAnimationOnImplThread() override;
   void OnDrawForOutputSurface() override;