ui: Forward declare DefaultSingletonTraits in header files.

ui/base/x/root_window_property_watcher_x.h

 // Copyright (c) 2011 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_ROOT_WINDOW_PROPERTY_WATCHER_X_H_
 #define UI_BASE_X_ROOT_WINDOW_PROPERTY_WATCHER_X_H_
 
 #include <gdk/gdk.h>
 
 #include "base/basictypes.h"
-#include "base/memory/singleton.h"
 #include "ui/base/gtk/gtk_signal.h"
 #include "ui/base/ui_export.h"
 
+template <typename T> struct DefaultSingletonTraits;
+
 namespace ui {
 
 namespace internal {
 
 // This class keeps track of changes to properties on the root window. This is
 // not to be used directly. Implement a watcher for the specific property you're
 // interested in.
 class RootWindowPropertyWatcherX {
  public:
   static RootWindowPropertyWatcherX* GetInstance();
 
  private:
   friend struct DefaultSingletonTraits<RootWindowPropertyWatcherX>;
 
   RootWindowPropertyWatcherX();
   ~RootWindowPropertyWatcherX();
 
   // Callback for PropertyChange XEvents.
   CHROMEG_CALLBACK_1(RootWindowPropertyWatcherX, GdkFilterReturn,
                      OnWindowXEvent, GdkXEvent*, GdkEvent*);
 
   DISALLOW_COPY_AND_ASSIGN(RootWindowPropertyWatcherX);
 };
 
 }  // namespace internal
 
 }  // namespace ui
 
 #endif  // UI_BASE_X_ROOT_WINDOW_PROPERTY_WATCHER_X_H_