[Sync] Remove ModelError::IsSet() in favor of base::Optional.

components/sync/model/mock_model_type_store.cc

Index: components/sync/model/mock_model_type_store.cc
diff --git a/components/sync/model/mock_model_type_store.cc b/components/sync/model/mock_model_type_store.cc
index 4c5a08fb46861d8a7f443c5449759c0788c2b43c..7b16e94ec626fe65ca71bef2da5a40089a77374b 100644
--- a/components/sync/model/mock_model_type_store.cc
+++ b/components/sync/model/mock_model_type_store.cc
@@ -47,7 +47,7 @@ void MockModelTypeStore::ReadAllMetadata(const ReadMetadataCallback& callback) {
     read_all_metadata_handler_.Run(callback);
   } else {
     base::ThreadTaskRunnerHandle::Get()->PostTask(
-        FROM_HERE, base::Bind(callback, ModelError(),
+        FROM_HERE, base::Bind(callback, base::Optional<ModelError>(),
                               base::Passed(std::unique_ptr<MetadataBatch>())));
   }
 }