[SyncFS] Make routines using metadata database async

chrome/browser/sync_file_system/drive_backend/sync_worker.h

Index: chrome/browser/sync_file_system/drive_backend/sync_worker.h
diff --git a/chrome/browser/sync_file_system/drive_backend/sync_worker.h b/chrome/browser/sync_file_system/drive_backend/sync_worker.h
index 357e77386dbe59bf9b4d29857a37891cd5b442fc..55ce5516f30d2d029f0abdb3c8f5479c41348f2b 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_worker.h
+++ b/chrome/browser/sync_file_system/drive_backend/sync_worker.h
@@ -105,6 +105,7 @@ class SyncWorker : public SyncTaskManager::Client {
   scoped_ptr<base::ListValue> DumpFiles(const GURL& origin);
   scoped_ptr<base::ListValue> DumpDatabase();
   void SetSyncEnabled(bool enabled);
+  void PromoteDemotedChanges();
   SyncStatusCode SetDefaultConflictResolutionPolicy(
       ConflictResolutionPolicy policy);
   SyncStatusCode SetConflictResolutionPolicy(
@@ -137,9 +138,6 @@ class SyncWorker : public SyncTaskManager::Client {
   void AddObserver(Observer* observer);
 
  private:
-  friend class DriveBackendSyncTest;
-  friend class SyncEngineTest;
-
   void DoDisableApp(const std::string& app_id,
                     const SyncStatusCallback& callback);
   void DoEnableApp(const std::string& app_id,