bookmarks: Forward declare DefaultSingletonTraits in bookmark_model_factory.h

chrome/browser/bookmarks/bookmark_model_factory.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 "chrome/browser/bookmarks/bookmark_model_factory.h"
 
+#include "base/memory/singleton.h"
 #include "chrome/browser/bookmarks/bookmark_model.h"
 #include "chrome/browser/history/history_service_factory.h"
 #include "chrome/browser/prefs/pref_service.h"
 #include "chrome/browser/profiles/profile_dependency_manager.h"
 #include "chrome/common/pref_names.h"
 
 // static
 BookmarkModel* BookmarkModelFactory::GetForProfile(Profile* profile) {
   return static_cast<BookmarkModel*>(
       GetInstance()->GetServiceForProfile(profile, true));
@@ skipping 32 matching lines @@
                           PrefService::UNSYNCABLE_PREF);
 }
 
 bool BookmarkModelFactory::ServiceRedirectedInIncognito() {
   return true;
 }
 
 bool BookmarkModelFactory::ServiceIsNULLWhileTesting() {
   return true;
 }