[SyncFS] Construct and destruct sync_worker in worker_task_runner

chrome/browser/sync_file_system/drive_backend/sync_engine.h

Index: chrome/browser/sync_file_system/drive_backend/sync_engine.h
diff --git a/chrome/browser/sync_file_system/drive_backend/sync_engine.h b/chrome/browser/sync_file_system/drive_backend/sync_engine.h
index 9f9329146e52b45593fd70296e47a1895a1b1e28..a907ee332654681899b9333c54602393bdba5e1c 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_engine.h
+++ b/chrome/browser/sync_file_system/drive_backend/sync_engine.h
@@ -67,6 +67,7 @@ class SyncEngine : public RemoteFileSyncService,
 
   void Initialize(const base::FilePath& base_dir,
                   base::SequencedTaskRunner* file_task_runner,
+                  const base::Closure& closure,
                   leveldb::Env* env_override);
 
   // RemoteFileSyncService overrides.
@@ -144,6 +145,8 @@ class SyncEngine : public RemoteFileSyncService,
              SigninManagerBase* signin_manager);
 
   void UpdateRegisteredApps();
+  void SetSyncWorker(const base::Closure& closure,
+                     scoped_ptr<SyncWorker> sync_worker);
 
   scoped_ptr<drive::DriveServiceInterface> drive_service_;
   scoped_ptr<DriveServiceWrapper> drive_service_wrapper_;