[Sync] Make directory types registration explicit in ModelTypeRegistry

components/sync/driver/data_type_controller.h

Index: components/sync/driver/data_type_controller.h
diff --git a/components/sync/driver/data_type_controller.h b/components/sync/driver/data_type_controller.h
index aef685cdbc2cb6afcf85026f484a8db39d22830a..840113211c64873aba7252a50dca6e8b2e192928 100644
--- a/components/sync/driver/data_type_controller.h
+++ b/components/sync/driver/data_type_controller.h
@@ -91,6 +91,12 @@ class DataTypeController : public base::SupportsWeakPtr<DataTypeController> {
   // return false while USS datatypes should return true.
   virtual bool ShouldLoadModelBeforeConfigure() const = 0;
 
+  // Called right before LoadModels. This method allows controller to register
+  // with sync engine. Directory datatypes download initial data in parallel
+  // with LoadModels and thus should be ready to receive updates with initial
+  // data before LoadModels finishes.
+  virtual void BeforeLoadModels(ModelTypeConfigurer* configurer) = 0;
+
   // Begins asynchronous operation of loading the model to get it ready for
   // model association. Once the models are loaded the callback will be invoked
   // with the result. If the models are already loaded it is safe to call the