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

ash/system/tray_accessibility.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_ACCESSIBILITY_H_
 #define ASH_SYSTEM_TRAY_ACCESSIBILITY_H_
 
 #include "ash/system/tray/tray_image_item.h"
 
 namespace views {
 class ImageView;
 class View;
 }
 
 namespace ash {
 
 class ASH_EXPORT AccessibilityObserver {
  public:
   virtual ~AccessibilityObserver() {}
 
   // Notifies when accessibilty mode changes.
   virtual void OnAccessibilityModeChanged(bool enabled) = 0;
 };
 
 namespace internal {
 
 class TrayAccessibility : public TrayImageItem,
                           public AccessibilityObserver {
  public:
-  TrayAccessibility();
+  explicit TrayAccessibility(SystemTray* system_tray);
   virtual ~TrayAccessibility();
 
  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 DestroyDefaultView() OVERRIDE;
   virtual void DestroyDetailedView() OVERRIDE;
 
   // Overridden from AccessibilityObserver.
   virtual void OnAccessibilityModeChanged(bool enabled) OVERRIDE;
 
   views::View* default_;
   views::View* detailed_;
 
   DISALLOW_COPY_AND_ASSIGN(TrayAccessibility);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_TRAY_ACCESSIBILITY_H_