ui: Forward declare DefaultSingletonTraits in header files.

ui/base/x/root_window_property_watcher_x.cc

 // 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.
 
 #include "ui/base/x/root_window_property_watcher_x.h"
 
 #include <gdk/gdk.h>
 #include <gdk/gdkx.h>
 
+#include "base/memory/singleton.h"
 #include "ui/base/x/active_window_watcher_x.h"
 #include "ui/base/x/work_area_watcher_x.h"
 
 namespace ui {
 
 namespace internal {
 
 // static
 RootWindowPropertyWatcherX* RootWindowPropertyWatcherX::GetInstance() {
   return Singleton<RootWindowPropertyWatcherX>::get();
@@ skipping 29 matching lines @@
     else if (xev->xproperty.atom == WorkAreaWatcherX::GetPropertyAtom())
       WorkAreaWatcherX::Notify();
   }
 
   return GDK_FILTER_CONTINUE;
 }
 
 }  // namespace internal
 
 }  // namespace ui