Let SyncBackupManager keep backup data in memory until shutdown. Only persist

sync/internal_api/sync_rollback_manager.cc

Index: sync/internal_api/sync_rollback_manager.cc
diff --git a/sync/internal_api/sync_rollback_manager.cc b/sync/internal_api/sync_rollback_manager.cc
index a26e54ec38a0db2907cd726a73b0d6e637a518f1..03062e60228f6072b485b49efcd45cd74591290d 100644
--- a/sync/internal_api/sync_rollback_manager.cc
+++ b/sync/internal_api/sync_rollback_manager.cc
@@ -42,6 +42,7 @@ void SyncRollbackManager::Init(
   if (SyncRollbackManagerBase::InitInternal(
           database_location,
           internal_components_factory,
+          InternalComponentsFactory::STORAGE_ON_DISK,
           unrecoverable_error_handler.Pass(),
           report_unrecoverable_error_function)) {
     change_delegate_ = change_delegate;