ui: Forward declare DefaultSingletonTraits in header files.

ui/views/widget/x11_desktop_handler.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_VIEWS_WIDGET_X11_DESKTOP_HANDLER_H_
 #define UI_VIEWS_WIDGET_X11_DESKTOP_HANDLER_H_
 
 #include <X11/Xlib.h>
 // Get rid of a macro from Xlib.h that conflicts with Aura's RootWindow class.
 #undef RootWindow
 
 #include "base/message_loop.h"
 #include "ui/aura/env_observer.h"
 #include "ui/aura/x11_atom_cache.h"
 #include "ui/views/views_export.h"
 
+template <typename T> struct DefaultSingletonTraits;
+
 namespace aura {
 class FocusManager;
 class DesktopActivationClient;
 }
 
 namespace views {
 
 // A singleton that owns global objects related to the desktop and listens for
 // X11 events on the X11 root window. Destroys itself when aura::Env is
 // deleted.
@@ skipping 35 matching lines @@
   // Global focus/activation managers.
   scoped_ptr<aura::FocusManager> focus_manager_;
   scoped_ptr<aura::DesktopActivationClient> desktop_activation_client_;
 
   DISALLOW_COPY_AND_ASSIGN(X11DesktopHandler);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_X11_DESKTOP_HANDLER_H_