[Sync] Introduce ModelError for USS error handling.

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 73b512c3ea0a96cb2026ae22cd79ac07c6f48524..60ea086eb11142187ccdd987757d8de7b741f884 100644
--- a/components/sync/model/mock_model_type_store.cc
+++ b/components/sync/model/mock_model_type_store.cc
@@ -15,9 +15,8 @@
 
 namespace syncer {
 
-MockModelTypeStore::MockModelTypeStore() {}
-
-MockModelTypeStore::~MockModelTypeStore() {}
+MockModelTypeStore::MockModelTypeStore() = default;
+MockModelTypeStore::~MockModelTypeStore() = default;
 
 void MockModelTypeStore::ReadData(const IdList& id_list,
                                   const ReadDataCallback& callback) {
@@ -46,7 +45,7 @@ void MockModelTypeStore::ReadAllMetadata(const ReadMetadataCallback& callback) {
     read_all_metadata_handler_.Run(callback);
   } else {
     base::ThreadTaskRunnerHandle::Get()->PostTask(
-        FROM_HERE, base::Bind(callback, SyncError(),
+        FROM_HERE, base::Bind(callback, ModelError(),
                               base::Passed(std::unique_ptr<MetadataBatch>())));
   }
 }