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

ui/gfx/win/singleton_hwnd.h

Index: ui/gfx/win/singleton_hwnd.h
diff --git a/ui/gfx/win/singleton_hwnd.h b/ui/gfx/win/singleton_hwnd.h
index 50fb9fd9df9e0d36187cbcfdbb5cc385c6588641..bdc698d2f3b40586330bd93acb25e17e1915fbd6 100644
--- a/ui/gfx/win/singleton_hwnd.h
+++ b/ui/gfx/win/singleton_hwnd.h
@@ -11,6 +11,7 @@
 #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;
@@ -52,6 +53,7 @@ class GFX_EXPORT SingletonHwnd : public WindowImpl {
 
   // List of registered observers.
   ObserverList<Observer> observer_list_;
+  base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(SingletonHwnd);
 };