ui: Forward declare DefaultSingletonTraits in header files.

ui/base/x/active_window_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_ACTIVE_WINDOW_WATCHER_X_H_
 #define UI_BASE_X_ACTIVE_WINDOW_WATCHER_X_H_
 
 #include "base/basictypes.h"
-#include "base/memory/singleton.h"
 #include "base/observer_list.h"
 #include "ui/base/ui_export.h"
 #include "ui/base/x/x11_util.h"
 
+template <typename T> struct DefaultSingletonTraits;
+
 namespace ui {
 
 class ActiveWindowWatcherXObserver;
 
 namespace internal {
 class RootWindowPropertyWatcherX;
 }
 
 // This is a helper class that is used to keep track of which window the X
 // window manager thinks is active. Add an Observer to listen for changes to
@@ skipping 26 matching lines @@
   void NotifyActiveWindowChanged();
 
   ObserverList<ActiveWindowWatcherXObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(ActiveWindowWatcherX);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_X_ACTIVE_WINDOW_WATCHER_X_H_