chrome: Cleanup class/struct forward declarations

chrome/browser/sync_file_system/drive_backend/sync_engine.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_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_H_
 
 #include <memory>
 #include <set>
 #include <string>
@@ skipping 34 matching lines @@
 
 namespace sync_file_system {
 
 class RemoteChangeProcessor;
 class SyncFileSystemTest;
 
 namespace drive_backend {
 
 class DriveServiceWrapper;
 class DriveUploaderWrapper;
-class MetadataDatabase;
 class RemoteChangeProcessorOnWorker;
 class RemoteChangeProcessorWrapper;
-class SyncTaskManager;
 class SyncWorkerInterface;
 
 class SyncEngine : public RemoteFileSyncService,
                    public LocalChangeProcessor,
                    public drive::DriveNotificationObserver,
                    public drive::DriveServiceObserver,
                    public net::NetworkChangeNotifier::NetworkChangeObserver,
                    public SigninManagerBase::Observer {
  public:
   typedef RemoteFileSyncService::Observer SyncServiceObserver;
@@ skipping 163 matching lines @@
   CallbackTracker callback_tracker_;
 
   base::WeakPtrFactory<SyncEngine> weak_ptr_factory_;
   DISALLOW_COPY_AND_ASSIGN(SyncEngine);
 };
 
 }  // namespace drive_backend
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_SYNC_ENGINE_H_