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

ash/system/brightness/tray_brightness.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_BRIGHTNESS_TRAY_BRIGHTNESS_H_
 #define ASH_SYSTEM_BRIGHTNESS_TRAY_BRIGHTNESS_H_
 
 #include "ash/system/brightness/brightness_observer.h"
 #include "ash/system/tray/system_tray_item.h"
 #include "base/compiler_specific.h"
 #include "base/memory/weak_ptr.h"
 
 namespace ash {
 namespace internal {
 
 namespace tray {
 class BrightnessView;
 }
 
 class TrayBrightness : public SystemTrayItem,
                        public BrightnessObserver {
  public:
-  TrayBrightness();
+  explicit TrayBrightness(SystemTray* system_tray);
   virtual ~TrayBrightness();
 
  private:
   // Sends a request to get the current screen brightness so |current_percent_|
   // can be initialized.
   void GetInitialBrightness();
 
   // Updates |current_percent_| with the initial brightness requested by
   // GetInitialBrightness(), if we haven't seen the brightness already in the
   // meantime.
@@ skipping 29 matching lines @@
   // Has |current_percent_| been initialized?
   bool got_current_percent_;
 
   DISALLOW_COPY_AND_ASSIGN(TrayBrightness);
 };
 
 }  // namespace internal
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_BRIGHTNESS_TRAY_BRIGHTNESS_H_