Tighter sync and faster ACK on SwapBuffers/PostSubBuffers

ui/compositor/compositor_observer.h

Index: ui/compositor/compositor_observer.h
diff --git a/ui/compositor/compositor_observer.h b/ui/compositor/compositor_observer.h
index e46010177f725f55f5a6fcbe68227abae231476a..992448a95ae09265c2200bf7ddaf585eae6f6160 100644
--- a/ui/compositor/compositor_observer.h
+++ b/ui/compositor/compositor_observer.h
@@ -19,6 +19,10 @@ class COMPOSITOR_EXPORT CompositorObserver {
   // account and has issued the graphics commands.
   virtual void OnCompositingStarted(Compositor* compositor) = 0;
 
+  // Called when compositing will complete: the present to the screen has been
+  // issued, but may not have completed yet.
+  virtual void OnCompositingWillEnd(Compositor* compositor) = 0;
+
   // Called when compositing completes: the present to screen has completed.
   virtual void OnCompositingEnded(Compositor* compositor) = 0;