Read compositor VSync information from platform, when possible

ash/display/display_change_observer_chromeos.h

 // Copyright 2013 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 ASH_DISPLAY_DISPLAY_CHANGE_OBSERVER_CHROMEOS_H
 #define ASH_DISPLAY_DISPLAY_CHANGE_OBSERVER_CHROMEOS_H
 
 #include "ash/ash_export.h"
 #include "ash/shell_observer.h"
 #include "base/basictypes.h"
@@ skipping 16 matching lines @@
       const chromeos::OutputConfigurator::OutputSnapshot& output);
 
   DisplayChangeObserver();
   virtual ~DisplayChangeObserver();
 
   // chromeos::OutputConfigurator::StateController overrides:
   virtual chromeos::OutputState GetStateForDisplayIds(
       const std::vector<int64>& outputs) const OVERRIDE;
   virtual bool GetResolutionForDisplayId(int64 display_id,
                                          int* width,
-                                         int* height) const OVERRIDE;
+                                         int* height,
+                                         float* refresh_rate) const OVERRIDE;
 
   // Overriden from chromeos::OutputConfigurator::Observer:
   virtual void OnDisplayModeChanged(
       const std::vector<chromeos::OutputConfigurator::OutputSnapshot>& outputs)
       OVERRIDE;
 
   // Overriden from ShellObserver:
   virtual void OnAppTerminating() OVERRIDE;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(DisplayChangeObserver);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_DISPLAY_AURA_DISPLAY_CHANGE_OBSERVER_CHROMEOS_H