ui: Forward declare DefaultSingletonTraits in header files.

ui/base/gtk/g_object_destructor_filo.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/gtk/g_object_destructor_filo.h"
 
 #include <glib-object.h>
+
 #include "base/logging.h"
+#include "base/memory/singleton.h"
 
 namespace ui {
 
 GObjectDestructorFILO::GObjectDestructorFILO() {
 }
 
 GObjectDestructorFILO::~GObjectDestructorFILO() {
   // Probably CHECK(handler_map_.empty()) would look natural here. But
   // some tests (some views_unittests) violate this assertion.
 }
@@ skipping 59 matching lines @@
   HandlerList dtors;
   iter->second.swap(dtors);
   handler_map_.erase(iter);
 
   // Execute hooks in local list in FILO order.
   for (HandlerList::iterator i = dtors.begin(); i != dtors.end(); ++i)
     i->callback(i->context, where_the_object_was);
 }
 
 }  // napespace ui