ui: Forward declare DefaultSingletonTraits in header files.

ui/views/focus/view_storage.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/views/focus/view_storage.h"
 
 #include <algorithm>
 
 #include "base/logging.h"
+#include "base/memory/singleton.h"
 #include "base/stl_util.h"
 
 namespace views {
 
 // static
 ViewStorage* ViewStorage::GetInstance() {
   return Singleton<ViewStorage>::get();
 }
 
 ViewStorage::ViewStorage() : view_storage_next_id_(0) {
@@ skipping 86 matching lines @@
     ids->erase(id_iter);
   }
 
   if (ids->empty()) {
     delete ids;
     view_to_ids_.erase(ids_iter);
   }
 }
 
 }  // namespace views