ui: Cleanup class/struct forward declarations

ui/app_list/search/history_data_store.h

 // Copyright 2013 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_APP_LIST_SEARCH_HISTORY_DATA_STORE_H_
 #define UI_APP_LIST_SEARCH_HISTORY_DATA_STORE_H_
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "ui/app_list/app_list_export.h"
 #include "ui/app_list/search/dictionary_data_store.h"
 #include "ui/app_list/search/history_data.h"
 
 namespace base {
 class DictionaryValue;
-class SequencedTaskRunner;
 }
 
 namespace app_list {
 
 namespace test {
 class HistoryDataStoreTest;
 }
 
 // A simple json store to persist HistoryData.
 class APP_LIST_EXPORT HistoryDataStore
@@ skipping 46 matching lines @@
   // on-memory data.
   std::unique_ptr<base::DictionaryValue> cached_dict_;
   scoped_refptr<DictionaryDataStore> data_store_;
 
   DISALLOW_COPY_AND_ASSIGN(HistoryDataStore);
 };
 
 }  // namespace app_list
 
 #endif  // UI_APP_LIST_SEARCH_HISTORY_DATA_STORE_H_