[Sync] Implement support for untracking new entities

components/sync/model/fake_model_type_change_processor.h

Index: components/sync/model/fake_model_type_change_processor.h
diff --git a/components/sync/model/fake_model_type_change_processor.h b/components/sync/model/fake_model_type_change_processor.h
index 309c41f7e0d7549728947817d15e6243f2dadb52..84f784642d9733054c68d5ff1f3a8989a00e0f14 100644
--- a/components/sync/model/fake_model_type_change_processor.h
+++ b/components/sync/model/fake_model_type_change_processor.h
@@ -36,6 +36,7 @@ class FakeModelTypeChangeProcessor : public ModelTypeChangeProcessor {
   void UpdateStorageKey(const EntityData& entity_data,
                         const std::string& storage_key,
                         MetadataChangeList* metadata_change_list) override;
+  void UntrackEntity(const EntityData& entity_data) override;
   void ModelReadyToSync(std::unique_ptr<MetadataBatch> batch) override;
   void OnSyncStarting(const ModelErrorHandler& error_handler,
                       const StartCallback& callback) override;