[Sync] Register USS types with ModelTypeRegistry before download

components/sync_driver/backend_data_type_configurer.h

Index: components/sync_driver/backend_data_type_configurer.h
diff --git a/components/sync_driver/backend_data_type_configurer.h b/components/sync_driver/backend_data_type_configurer.h
index 77ca6150af1f1a6bc678922fe815f243c764b796..6b541d6947e38041174c71617e718a6ddafddc5a 100644
--- a/components/sync_driver/backend_data_type_configurer.h
+++ b/components/sync_driver/backend_data_type_configurer.h
@@ -77,7 +77,7 @@ class BackendDataTypeConfigurer {
   virtual void DeactivateDirectoryDataType(syncer::ModelType type) = 0;
 
   // Activates change processing for the given non-blocking data type.
-  // This must be called synchronously with the data type's model association.
+  // This must be called before initial sync for data type.
   virtual void ActivateNonBlockingDataType(
       syncer::ModelType type,
       scoped_ptr<syncer_v2::ActivationContext> activation_context) = 0;