[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..aad38e195a4254c7549a49205ee0b05d98f36bd7 100644
--- a/chrome/browser/sync_file_system/drive_backend/sync_worker.h
+++ b/chrome/browser/sync_file_system/drive_backend/sync_worker.h
@@ -69,6 +69,7 @@ class SyncWorker : public SyncTaskManager::Client {
                                      SyncDirection direction) = 0;
     virtual void UpdateServiceState(RemoteServiceState state,
                                     const std::string& description) = 0;
+    virtual void OnPromoteDemotedChanges(size_t num_trackers) = 0;
 
    protected:
     virtual ~Observer() {}
@@ -105,6 +106,7 @@ 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(