ui: Forward declare DefaultSingletonTraits in header files.

ui/base/x/work_area_watcher_x.cc

 // 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.
 
 #include "ui/base/x/work_area_watcher_x.h"
 
+#include "base/memory/singleton.h"
 #include "ui/base/work_area_watcher_observer.h"
 #include "ui/base/x/root_window_property_watcher_x.h"
 #include "ui/base/x/x11_util.h"
 
 namespace ui {
 
 static const char* const kNetWorkArea = "_NET_WORKAREA";
 
 // static
 WorkAreaWatcherX* WorkAreaWatcherX::GetInstance() {
@@ skipping 26 matching lines @@
 }
 
 WorkAreaWatcherX::~WorkAreaWatcherX() {
 }
 
 void WorkAreaWatcherX::NotifyWorkAreaChanged() {
   FOR_EACH_OBSERVER(WorkAreaWatcherObserver, observers_, WorkAreaChanged());
 }
 
 }  // namespace ui