[Sync] Implement support for untracking new entities

components/sync/model/fake_model_type_change_processor.cc

Index: components/sync/model/fake_model_type_change_processor.cc
diff --git a/components/sync/model/fake_model_type_change_processor.cc b/components/sync/model/fake_model_type_change_processor.cc
index 6ca9f46fb7123cb61166e5bf67829852f4d37641..371cfeb99eab712ce59f12641a9f0b16fdd77619 100644
--- a/components/sync/model/fake_model_type_change_processor.cc
+++ b/components/sync/model/fake_model_type_change_processor.cc
@@ -40,6 +40,9 @@ void FakeModelTypeChangeProcessor::UpdateStorageKey(
     const std::string& storage_key,
     MetadataChangeList* metadata_change_list) {}
 
+void FakeModelTypeChangeProcessor::UntrackEntity(
+    const EntityData& entity_data) {}
+
 void FakeModelTypeChangeProcessor::ModelReadyToSync(
     std::unique_ptr<MetadataBatch> batch) {}