[SyncFS] Remove usage of file task runner under drive_backend/

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 1493e0b3b36d62e2de5734599ae34b8db66805d6..8fb91a39ded43f57bbf0b4c8af7bb468a17aef77 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_engine.h
+++ b/chrome/browser/sync_file_system/drive_backend/sync_engine.h
@@ -148,7 +148,6 @@ class SyncEngine : public RemoteFileSyncService,
 
   SyncEngine(base::SingleThreadTaskRunner* ui_task_runner,
              base::SequencedTaskRunner* worker_task_runner,
-             base::SequencedTaskRunner* file_task_runner,
              base::SequencedTaskRunner* drive_task_runner,
              const base::FilePath& sync_file_system_dir,
              TaskLogger* task_logger,
@@ -172,7 +171,6 @@ class SyncEngine : public RemoteFileSyncService,
 
   scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner_;
   scoped_refptr<base::SequencedTaskRunner> worker_task_runner_;
-  scoped_refptr<base::SequencedTaskRunner> file_task_runner_;
   scoped_refptr<base::SequencedTaskRunner> drive_task_runner_;
 
   const base::FilePath sync_file_system_dir_;