Add the "virtual" keyword on method overrides that are missing it.

app/hi_res_timer_manager.h

 // Copyright (c) 2009 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 APP_HI_RES_TIMER_MANAGER_H_
 #define APP_HI_RES_TIMER_MANAGER_H_
 #pragma once
 
 #include "app/system_monitor.h"
 
 // Ensures that the Windows high resolution timer is only used
 // when not running on battery power.
 class HighResolutionTimerManager : public SystemMonitor::PowerObserver {
  public:
   HighResolutionTimerManager();
   virtual ~HighResolutionTimerManager();
 
   // SystemMonitor::PowerObserver:
-  void OnPowerStateChange(bool on_battery_power);
+  virtual void OnPowerStateChange(bool on_battery_power);
 
  private:
   // Enable or disable the faster multimedia timer.
   void UseHiResClock(bool use);
 
   bool hi_res_clock_used_;
 
   DISALLOW_COPY_AND_ASSIGN(HighResolutionTimerManager);
 };
 
 #endif  // APP_HI_RES_TIMER_MANAGER_H_