[Win] Battery Status API: use SingletonHwnd for notifications.

ui/gfx/win/singleton_hwnd.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 UI_GFX_WIN_SINGLETON_HWND_H_
 #define UI_GFX_WIN_SINGLETON_HWND_H_
 
 #include <windows.h>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
 #include "base/observer_list.h"
+#include "base/threading/thread_checker.h"
 #include "ui/gfx/win/window_impl.h"
 
 template<typename T> struct DefaultSingletonTraits;
 
 namespace gfx {
 
 // Singleton message-only HWND that allows interested clients to receive WM_*
 // notifications.
 class GFX_EXPORT SingletonHwnd : public WindowImpl {
  public:
@@ skipping 21 matching lines @@
                                     DWORD msg_map_id) OVERRIDE;
 
  private:
   friend struct DefaultSingletonTraits<SingletonHwnd>;
 
   SingletonHwnd();
   ~SingletonHwnd();
 
   // List of registered observers.
   ObserverList<Observer> observer_list_;
+  base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(SingletonHwnd);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_WIN_SINGLETON_HWND_H_