[Sync] Introduce ModelError for USS error handling.

components/reading_list/ios/reading_list_model_unittest.mm

Index: components/reading_list/ios/reading_list_model_unittest.mm
diff --git a/components/reading_list/ios/reading_list_model_unittest.mm b/components/reading_list/ios/reading_list_model_unittest.mm
index bf7c1b10697260c87a2f3faa98008d9fe53bb882..1a85a92edc04aa26e22340cf0d9b2d3029add1f6 100644
--- a/components/reading_list/ios/reading_list_model_unittest.mm
+++ b/components/reading_list/ios/reading_list_model_unittest.mm
@@ -109,18 +109,18 @@ class TestReadingListStorage : public ReadingListModelStorage {
     return std::unique_ptr<syncer::MetadataChangeList>();
   }
 
-  syncer::SyncError MergeSyncData(
+  syncer::ModelError MergeSyncData(
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityDataMap entity_data_map) override {
     NOTREACHED();
-    return syncer::SyncError();
+    return syncer::ModelError();
   }
 
-  syncer::SyncError ApplySyncChanges(
+  syncer::ModelError ApplySyncChanges(
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityChangeList entity_changes) override {
     NOTREACHED();
-    return syncer::SyncError();
+    return syncer::ModelError();
   }
 
   void GetData(StorageKeyList storage_keys, DataCallback callback) override {