ui: Forward declare DefaultSingletonTraits in header files.

ui/gfx/gtk_native_view_id_manager.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/gfx/gtk_native_view_id_manager.h"
 
+#include <gdk/gdkx.h>
 #include <gtk/gtk.h>
-#include <gdk/gdkx.h>
 
 #include "base/logging.h"
+#include "base/memory/singleton.h"
 #include "base/rand_util.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/gfx/gtk_preserve_window.h"
 
 // -----------------------------------------------------------------------------
 // Bounce functions for GTK to callback into a C++ object...
 
 void OnRealize(gfx::NativeView widget, void* arg) {
   GtkNativeViewManager* manager = reinterpret_cast<GtkNativeViewManager*>(arg);
   manager->OnRealize(widget);
 }
 
@@ skipping 220 matching lines @@
 
     if (k != perm_xid_to_info_.end())
       k->second.widget = NULL;
   }
 
   native_view_to_id_.erase(i);
   id_to_info_.erase(j);
 }
 
 // -----------------------------------------------------------------------------