[sync] Set up notification-free infra for triggering local sync refresh

chrome/browser/sync/glue/sync_backend_host.h

Index: chrome/browser/sync/glue/sync_backend_host.h
diff --git a/chrome/browser/sync/glue/sync_backend_host.h b/chrome/browser/sync/glue/sync_backend_host.h
index d78de18b8500e6a38433df56d31ea275e3e663db..2ba4d5bdea735728344edd07d1475e4ebe1f5da8 100644
--- a/chrome/browser/sync/glue/sync_backend_host.h
+++ b/chrome/browser/sync/glue/sync_backend_host.h
@@ -77,6 +77,9 @@ class SyncBackendHost : public sync_driver::BackendDataTypeConfigurer {
       scoped_ptr<syncer::SyncEncryptionHandler::NigoriState>
           saved_nigori_state) = 0;
 
+  // Called on the frontend's thread to trigger a refresh.
+  virtual void TriggerRefresh(const syncer::ModelTypeSet& types) = 0;
+
   // Called on the frontend's thread to update SyncCredentials.
   virtual void UpdateCredentials(
       const syncer::SyncCredentials& credentials) = 0;