[Sync] Make directory types registration explicit in ModelTypeRegistry

components/sync/driver/model_association_manager.h

Index: components/sync/driver/model_association_manager.h
diff --git a/components/sync/driver/model_association_manager.h b/components/sync/driver/model_association_manager.h
index 2f8bd9d9a237fa01c0e584b009bc56ab92a78ee1..c083d4701e2cef81e864c05df0c432bee4ba1a89 100644
--- a/components/sync/driver/model_association_manager.h
+++ b/components/sync/driver/model_association_manager.h
@@ -28,6 +28,10 @@ class DataTypeController;
 // those operations are passed back via this interface.
 class ModelAssociationManagerDelegate {
  public:
+  // Called right before ModelAssociationManager calls LoadModels. Allows
+  // directory types to register with sync engine before download starts.
+  virtual void OnSingleDataTypeWillStart(ModelType type) = 0;
+
   // Called when all desired types are ready to be configured with
   // ModelTypeConfigurer. Data type is ready when its progress marker is
   // available to configurer. Directory data types are always ready, their