ui: Forward declare DefaultSingletonTraits in header files.

ui/base/gtk/g_object_destructor_filo.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_GTK_G_OBJECT_DESTRUCTOR_FILO_H_
 #define UI_BASE_GTK_G_OBJECT_DESTRUCTOR_FILO_H_
 
 #include <glib.h>
+#include <list>
 #include <map>
-#include <list>
 
-#include "base/memory/singleton.h"
+#include "base/basictypes.h"
 #include "ui/base/ui_export.h"
 
+template <typename T> struct DefaultSingletonTraits;
+
 typedef struct _GObject GObject;
 
 namespace ui {
 
 // This class hooks calls to g_object_weak_ref()/unref() and executes them in
 // FILO order. This is important if there are several hooks to the single object
 // (set up at different levels of class hierarchy) and the lowest hook (set up
 // first) is deleting self - it must be called last (among hooks for the given
 // object). Unfortunately Glib does not provide this guarantee.
 //
@@ skipping 55 matching lines @@
   }
 
   HandlerMap handler_map_;
 
   DISALLOW_COPY_AND_ASSIGN(GObjectDestructorFILO);
 };
 
 }  // namespace ui
 
 #endif  // UI_BASE_GTK_G_OBJECT_DESTRUCTOR_FILO_H_