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

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 f5ee9c99cd11e0f223da2c21ef5df436179d3c96..bdcc008462f268f71095840916b18c48dd060831 100644
--- a/components/reading_list/ios/reading_list_model_unittest.mm
+++ b/components/reading_list/ios/reading_list_model_unittest.mm
@@ -110,18 +110,18 @@ class TestReadingListStorage : public ReadingListModelStorage {
     return std::unique_ptr<syncer::MetadataChangeList>();
   }
 
-  syncer::ModelError MergeSyncData(
+  base::Optional<syncer::ModelError> MergeSyncData(
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityDataMap entity_data_map) override {
     NOTREACHED();
-    return syncer::ModelError();
+    return {};
   }
 
-  syncer::ModelError ApplySyncChanges(
+  base::Optional<syncer::ModelError> ApplySyncChanges(
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityChangeList entity_changes) override {
     NOTREACHED();
-    return syncer::ModelError();
+    return {};
   }
 
   void GetData(StorageKeyList storage_keys, DataCallback callback) override {