[Sync] Register USS types with ModelTypeRegistry before download

sync/internal_api/public/engine/model_safe_worker.cc

Index: sync/internal_api/public/engine/model_safe_worker.cc
diff --git a/sync/internal_api/public/engine/model_safe_worker.cc b/sync/internal_api/public/engine/model_safe_worker.cc
index d43389d58fb08b326b667048bc20e71e5bec14c4..ea99532abc626d0ae9ab14f9ca14ce05ab149e7b 100644
--- a/sync/internal_api/public/engine/model_safe_worker.cc
+++ b/sync/internal_api/public/engine/model_safe_worker.cc
@@ -63,6 +63,8 @@ std::string ModelSafeGroupToString(ModelSafeGroup group) {
       return "GROUP_PASSIVE";
     case GROUP_PASSWORD:
       return "GROUP_PASSWORD";
+    case GROUP_NON_BLOCKING:
+      return "GROUP_NON_BLOCKING";
     default:
       NOTREACHED();
       return "INVALID";