[Sync] Register USS types with ModelTypeRegistry before download

components/sync_driver/proxy_data_type_controller.cc

Index: components/sync_driver/proxy_data_type_controller.cc
diff --git a/components/sync_driver/proxy_data_type_controller.cc b/components/sync_driver/proxy_data_type_controller.cc
index c04142dca3bee08f49a51decd9bd8cfc3d1f4b49..6a534e7d8686944e5d39037cff13263b12382704 100644
--- a/components/sync_driver/proxy_data_type_controller.cc
+++ b/components/sync_driver/proxy_data_type_controller.cc
@@ -30,6 +30,9 @@ void ProxyDataTypeController::LoadModels(
   model_load_callback.Run(type(), syncer::SyncError());
 }
 
+void ProxyDataTypeController::RegisterWithBackend(
+    BackendDataTypeConfigurer* configurer) {}
+
 void ProxyDataTypeController::StartAssociating(
     const StartCallback& start_callback) {
   syncer::SyncMergeResult local_merge_result(type_);