Move Singleton and related structs to namespace base

ui/base/x/x11_menu_list.h

 // Copyright (c) 2014 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_BASE_X_X11_MENU_LIST_H_
 #define UI_BASE_X_X11_MENU_LIST_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
 #include "ui/base/ui_base_export.h"
 #include "ui/gfx/x/x11_types.h"
 
 // A process wide singleton cache for X menus.
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 namespace ui {
 
 // Keeps track of created and destroyed top level menu windows.
 class UI_BASE_EXPORT XMenuList {
  public:
   static XMenuList* GetInstance();
 
   // Checks if |menu| has _NET_WM_WINDOW_TYPE property set to
   // "_NET_WM_WINDOW_TYPE_MENU" atom and if so caches it.
   void MaybeRegisterMenu(XID menu);
 
   // Finds |menu| in cache and if found removes it.
   void MaybeUnregisterMenu(XID menu);
 
   // Inserts cached menu XIDs at the beginning of |stack|.
   void InsertMenuWindowXIDs(std::vector<XID>* stack);
 
  private:
-  friend struct DefaultSingletonTraits<XMenuList>;
+  friend struct base::DefaultSingletonTraits<XMenuList>;
   XMenuList();
   ~XMenuList();
 
   std::vector<XID> menus_;
   XAtom menu_type_atom_;
   DISALLOW_COPY_AND_ASSIGN(XMenuList);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_X_X11_MENU_LIST_H_