Stop using shell::GetInstance()->system_tray() in system tray items

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 12 matching lines @@
 }
 
 enum IconSet {
   ICON_LIGHT,
   ICON_DARK
 };
 
 class TrayPower : public SystemTrayItem,
                   public PowerStatusObserver {
  public:
-  TrayPower();
+  explicit TrayPower(SystemTray* system_tray);
   virtual ~TrayPower();
 
   // 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);
 
@@ skipping 26 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_