[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 1b660683c2d322b958c6f90b4fc97e013477f338..f65ffa1a1de2ddff4ffc4b1bc26b217087b1c60e 100644
--- a/chrome/browser/sync/glue/sync_backend_host.h
+++ b/chrome/browser/sync/glue/sync_backend_host.h
@@ -76,6 +76,9 @@ 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;