ui: Forward declare DefaultSingletonTraits in header files.

ui/base/x/active_window_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/active_window_watcher_x.h"
 
 #include <gdk/gdk.h>
 #include <gdk/gdkx.h>
+
+#include "base/memory/singleton.h"
+#include "ui/base/gtk/gdk_x_compat.h"
 #include "ui/base/gtk/gtk_compat.h"
-#include "ui/base/gtk/gdk_x_compat.h"
-
 #include "ui/base/x/active_window_watcher_x_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 kNetActiveWindow = "_NET_ACTIVE_WINDOW";
 
 // static
 ActiveWindowWatcherX* ActiveWindowWatcherX::GetInstance() {
@@ skipping 67 matching lines @@
     GdkDisplay* display = gdk_display_get_default();
     GdkWindow* active_window = gdk_x11_window_lookup_for_display(display, xid);
     FOR_EACH_OBSERVER(ActiveWindowWatcherXObserver, observers_,
                       ActiveWindowChanged(active_window));
   }
   if (property)
     XFree(property);
 }
 
 }  // namespace ui