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 b4100c76136f7a83e03e6423ba23d6d5790e34b2..e7b8ea694c48e7253a66361007006208d9225e93 100644
--- a/sync/internal_api/sync_rollback_manager.cc
+++ b/sync/internal_api/sync_rollback_manager.cc
@@ -26,6 +26,7 @@ void SyncRollbackManager::Init(InitArgs* args) {
   if (SyncRollbackManagerBase::InitInternal(
           args->database_location,
           args->internal_components_factory.get(),
+          InternalComponentsFactory::STORAGE_ON_DISK,
           args->unrecoverable_error_handler.Pass(),
           args->report_unrecoverable_error_function)) {
     change_delegate_ = args->change_delegate;