chrome: Cleanup class/struct forward declarations

chrome/browser/sync_file_system/drive_backend/sync_engine_initializer.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 CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_INITIALIZER_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_INITIALIZER_H_
 
 #include <stdint.h>
 
 #include <memory>
 #include <string>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/sync_file_system/drive_backend/sync_task.h"
 #include "chrome/browser/sync_file_system/sync_callbacks.h"
 #include "google_apis/drive/drive_api_error_codes.h"
 #include "google_apis/drive/drive_common_callbacks.h"
 
-namespace drive {
-class DriveServiceInterface;
-}
-
 namespace google_apis {
 class AboutResource;
 class FileList;
 class FileResource;
 }
 
 namespace leveldb {
 class Env;
 }
 
@@ skipping 83 matching lines @@
 
   base::WeakPtrFactory<SyncEngineInitializer> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncEngineInitializer);
 };
 
 }  // namespace drive_backend
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_INITIALIZER_H_