Move Singleton and related structs to namespace base

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/observer_list.h"
 #include "ui/gfx/gfx_export.h"
 #include "ui/gfx/win/window_impl.h"
 
+namespace base {
 template<typename T> struct DefaultSingletonTraits;
+}
 
 namespace gfx {
 
 class SingletonHwndObserver;
 
 // Singleton message-only HWND that allows interested clients to receive WM_*
 // notifications.
 class GFX_EXPORT SingletonHwnd : public WindowImpl {
  public:
   static SingletonHwnd* GetInstance();
 
   // Windows callback for WM_* notifications.
   BOOL ProcessWindowMessage(HWND window,
                             UINT message,
                             WPARAM wparam,
                             LPARAM lparam,
                             LRESULT& result,
                             DWORD msg_map_id) override;
 
  private:
   friend class SingletonHwndObserver;
-  friend struct DefaultSingletonTraits<SingletonHwnd>;
+  friend struct base::DefaultSingletonTraits<SingletonHwnd>;
 
   SingletonHwnd();
   ~SingletonHwnd() override;
 
   // Add/remove SingletonHwndObserver to forward WM_* notifications.
   void AddObserver(SingletonHwndObserver* observer);
   void RemoveObserver(SingletonHwndObserver* observer);
 
   // List of registered observers.
   base::ObserverList<SingletonHwndObserver, true> observer_list_;
 
   DISALLOW_COPY_AND_ASSIGN(SingletonHwnd);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_WIN_SINGLETON_HWND_H_