Tighter sync and faster ACK on SwapBuffers/PostSubBuffers

ui/compositor/compositor_observer.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_COMPOSITOR_COMPOSITOR_OBSERVER_H_
 #define UI_COMPOSITOR_COMPOSITOR_OBSERVER_H_
 #pragma once
 
 #include "ui/compositor/compositor_export.h"
 
 namespace ui {
 
 class Compositor;
 
 // A compositor observer is notified when compositing completes.
 class COMPOSITOR_EXPORT CompositorObserver {
  public:
   // Called when compositing started: it has taken all the layer changes into
   // 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;
 
  protected:
   virtual ~CompositorObserver() {}
 };
 
 } // namespace ui
 
 #endif  // UI_COMPOSITOR_COMPOSITOR_OBSERVER_H_