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

ash/system/tray_update.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_TRAY_UPDATE_H_
 #define ASH_SYSTEM_TRAY_UPDATE_H_
 
 #include "ash/system/tray/tray_image_item.h"
 #include "ash/system/user/update_observer.h"
 #include "base/memory/scoped_ptr.h"
 
 namespace views {
 class View;
 }
 
 namespace ash {
 
+class SystemTray;
+
 namespace internal {
 
 namespace tray {
 class UpdateNagger;
 }
 
 class TrayUpdate : public TrayImageItem,
                    public UpdateObserver {
  public:
-  TrayUpdate();
+  explicit TrayUpdate(SystemTray* system_tray);
   virtual ~TrayUpdate();
 
  private:
   // Overridden from TrayImageItem.
   virtual bool GetInitialVisibility() OVERRIDE;
   virtual views::View* CreateDefaultView(user::LoginStatus status) OVERRIDE;
   virtual views::View* CreateDetailedView(user::LoginStatus status) OVERRIDE;
   virtual void DestroyDetailedView() OVERRIDE;
 
   // Overridden from UpdateObserver.
   virtual void OnUpdateRecommended(UpdateSeverity severity) OVERRIDE;
 
   // Used to nag the user in case the tray has been hidden too long with an
   // unseen update notification.
   scoped_ptr<tray::UpdateNagger> nagger_;
 
   UpdateObserver::UpdateSeverity severity_;
 
   DISALLOW_COPY_AND_ASSIGN(TrayUpdate);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_TRAY_UPDATE_H_