[Sync] Register USS types with ModelTypeRegistry before download

components/sync_driver/proxy_data_type_controller.h

Index: components/sync_driver/proxy_data_type_controller.h
diff --git a/components/sync_driver/proxy_data_type_controller.h b/components/sync_driver/proxy_data_type_controller.h
index d08aacd01f401c3809056318fcf55cff94a2c1b8..5e075e2b887b72b9cbec625d9a879a8965a6c17d 100644
--- a/components/sync_driver/proxy_data_type_controller.h
+++ b/components/sync_driver/proxy_data_type_controller.h
@@ -26,6 +26,7 @@ class ProxyDataTypeController : public DataTypeController {
   // DataTypeController interface.
   bool ShouldLoadModelBeforeConfigure() const override;
   void LoadModels(const ModelLoadCallback& model_load_callback) override;
+  void RegisterWithBackend(BackendDataTypeConfigurer* configurer) override;
   void StartAssociating(const StartCallback& start_callback) override;
   void Stop() override;
   syncer::ModelType type() const override;