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

ash/system/monitor/tray_monitor.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_DATE_TRAY_MONITOR_H_
 #define ASH_SYSTEM_DATE_TRAY_MONITOR_H_
 
 #include "ash/system/tray/system_tray_item.h"
 #include "base/timer.h"
 
 namespace views {
 class Label;
 }
 
 namespace ash {
+
+class SystemTray;
+
 namespace internal {
 
 class TrayMonitor : public SystemTrayItem {
  public:
-  TrayMonitor();
+  explicit TrayMonitor(SystemTray* system_tray);
   virtual ~TrayMonitor();
 
  private:
   // Overridden from SystemTrayItem.
   virtual views::View* CreateTrayView(user::LoginStatus status) OVERRIDE;
   virtual void DestroyTrayView() OVERRIDE;
 
   void RefreshStats();
 
   views::Label* label_;
   base::RepeatingTimer<TrayMonitor> refresh_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(TrayMonitor);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_DATE_TRAY_MONITOR_H_