chrome: Cleanup class/struct forward declarations

chrome/browser/extensions/api/storage/setting_sync_data.h

 // 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.
 
 #ifndef CHROME_BROWSER_EXTENSIONS_API_STORAGE_SETTING_SYNC_DATA_H_
 #define CHROME_BROWSER_EXTENSIONS_API_STORAGE_SETTING_SYNC_DATA_H_
 
 #include <memory>
 #include <string>
 
 #include "base/macros.h"
 #include "base/values.h"
 #include "components/sync/model/sync_change.h"
 
 namespace syncer {
 class SyncData;
 }
 
-namespace sync_pb {
-class ExtensionSettingSpecifics;
-}
-
 namespace extensions {
 
 // Container for data interpreted from sync data/changes for an extension or
 // app setting. Safe and efficient to copy.
 class SettingSyncData {
  public:
   // Creates from a sync change.
   explicit SettingSyncData(const syncer::SyncChange& sync_change);
 
   // Creates from sync data. |change_type| will be ACTION_INVALID.
@@ skipping 32 matching lines @@
   std::unique_ptr<base::Value> value_;
 
   DISALLOW_COPY_AND_ASSIGN(SettingSyncData);
 };
 
 using SettingSyncDataList = std::vector<std::unique_ptr<SettingSyncData>>;
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_STORAGE_SETTING_SYNC_DATA_H_