[SyncFS] Make routines using metadata database async

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 45bf4b344f379ca6fd622d7161cf8c0958ba235b..df02c229421b3ead9c3d493317d559a3801c21ec 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_engine.h
+++ b/chrome/browser/sync_file_system/drive_backend/sync_engine.h
@@ -121,7 +121,11 @@ class SyncEngine : public RemoteFileSyncService,
 
   drive::DriveServiceInterface* GetDriveService();
   drive::DriveUploaderInterface* GetDriveUploader();
-  MetadataDatabase* GetMetadataDatabase();
+
+  // NOTICE: Basically, metadata database runs on |worker_task_runner_|.
+  // Handling it through GetMetadataDatabaseForTesting() in the main thread
+  // may bring race condition.
+  MetadataDatabase* GetMetadataDatabaseForTesting();
   SyncTaskManager* GetSyncTaskManagerForTesting();
 
   void OnPendingFileListUpdated(int item_count);
@@ -145,7 +149,7 @@ class SyncEngine : public RemoteFileSyncService,
              ExtensionServiceInterface* extension_service,
              SigninManagerBase* signin_manager);
 
-  void UpdateRegisteredApps();
+  void UpdateRegisteredAppsForTesting();
 
   scoped_ptr<drive::DriveServiceInterface> drive_service_;
   scoped_ptr<DriveServiceWrapper> drive_service_wrapper_;