Only update system power indicators on change

ash/system/power/tray_power.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 ASH_SYSTEM_POWER_TRAY_POWER_H_
 #define ASH_SYSTEM_POWER_TRAY_POWER_H_
 
 #include "ash/system/power/power_status_observer.h"
 #include "ash/system/tray/system_tray_item.h"
 
@@ skipping 15 matching lines @@
   ICON_LIGHT,
   ICON_DARK
 };
 
 class TrayPower : public SystemTrayItem,
                   public PowerStatusObserver {
  public:
   TrayPower();
   virtual ~TrayPower();
 
-  // Gets battery image based on |supply_status|. If |supply_status| is
-  // uncertain about the power state, return |default_image| instead.
-  static gfx::ImageSkia GetBatteryImage(const PowerSupplyStatus& supply_status,
-                                        IconSet icon_set,
-                                        const gfx::ImageSkia& default_image);
+  // Gets the icon index in the battery icon array image based on
+  // |supply_status|.  If |supply_status| is uncertain about the power state,
+  // returns -1.
+  static int GetBatteryImageIndex(const PowerSupplyStatus& supply_status);
+
+  // Looks up the actual icon in the icon array image for |image_index|.
+  static gfx::ImageSkia GetBatteryImage(int image_index, IconSet icon_set);
 
  private:
   enum NotificationState {
     NOTIFICATION_NONE,
     NOTIFICATION_LOW_POWER,
     NOTIFICATION_CRITICAL
   };
 
   // Overridden from SystemTrayItem.
   virtual views::View* CreateTrayView(user::LoginStatus status) OVERRIDE;
@@ skipping 17 matching lines @@
   tray::PowerNotificationView* notification_view_;
   NotificationState notification_state_;
 
   DISALLOW_COPY_AND_ASSIGN(TrayPower);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_POWER_TRAY_POWER_H_