[Sync] Make directory types registration explicit in ModelTypeRegistry

components/sync/driver/directory_data_type_controller.h

Index: components/sync/driver/directory_data_type_controller.h
diff --git a/components/sync/driver/directory_data_type_controller.h b/components/sync/driver/directory_data_type_controller.h
index b4b2777894bfe0a1917aba93752286f1a84be48e..d667e45ae401ffc8f01cbb401b9f2db1ea7f0960 100644
--- a/components/sync/driver/directory_data_type_controller.h
+++ b/components/sync/driver/directory_data_type_controller.h
@@ -26,9 +26,12 @@ class DirectoryDataTypeController : public DataTypeController {
   void GetAllNodes(const AllNodesCallback& callback) override;
   void GetStatusCounters(const StatusCountersCallback& callback) override;
 
-  // Directory based data types don't need to register with backend.
-  // ModelTypeRegistry will create all necessary objects in
-  // SetEnabledDirectoryTypes based on routing info.
+  // Directory types need to register with sync engine before LoadModels because
+  // downloading initial data happens in parallel with LoadModels.
+  void BeforeLoadModels(ModelTypeConfigurer* configurer) override;
+
+  // Directory based data types register with backend before LoadModels in
+  // BeforeLoadModels. No need to do anything in RegisterWithBackend.
   void RegisterWithBackend(base::Callback<void(bool)> set_downloaded,
                            ModelTypeConfigurer* configurer) override;