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

ash/system/logout_button/tray_logout_button.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_LOGOUT_BUTTON_TRAY_LOGOUT_BUTTON_H_
 #define ASH_SYSTEM_LOGOUT_BUTTON_TRAY_LOGOUT_BUTTON_H_
 
 #include "ash/system/logout_button/logout_button_observer.h"
 #include "ash/system/tray/system_tray_item.h"
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 
 namespace ash {
 namespace internal {
 
 namespace tray {
 class LogoutButton;
 }
 
 // Adds a logout button to the system tray if enabled by the
 // kShowLogoutButtonInTray pref.
 class TrayLogoutButton : public SystemTrayItem, public LogoutButtonObserver {
  public:
-  TrayLogoutButton();
+  explicit TrayLogoutButton(SystemTray* system_tray);
 
   // Overridden from SystemTrayItem.
   virtual views::View* CreateTrayView(user::LoginStatus status) OVERRIDE;
   virtual void DestroyTrayView() OVERRIDE;
   virtual void UpdateAfterLoginStatusChange(user::LoginStatus status) OVERRIDE;
 
   // Overridden from LogoutButtonObserver.
   virtual void OnShowLogoutButtonInTrayChanged(bool show) OVERRIDE;
 
  private:
   tray::LogoutButton* logout_button_;
 
   DISALLOW_COPY_AND_ASSIGN(TrayLogoutButton);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_LOGOUT_BUTTON_TRAY_LOGOUT_BUTTON_H_