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

components/reading_list/ios/reading_list_store.h

Index: components/reading_list/ios/reading_list_store.h
diff --git a/components/reading_list/ios/reading_list_store.h b/components/reading_list/ios/reading_list_store.h
index 4eef45974d689ce88578e65a5e3c23a3ed06281b..7d454a2d64403ba1ff694f19ad2c419f1155a80c 100644
--- a/components/reading_list/ios/reading_list_store.h
+++ b/components/reading_list/ios/reading_list_store.h
@@ -55,7 +55,7 @@ class ReadingListStore : public ReadingListModelStorage,
   // combine all change atomically, should save the metadata after the data
   // changes, so that this merge will be re-driven by sync if is not completely
   // saved during the current run.
-  syncer::ModelError MergeSyncData(
+  base::Optional<syncer::ModelError> MergeSyncData(
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityDataMap entity_data_map) override;
 
@@ -64,7 +64,7 @@ class ReadingListStore : public ReadingListModelStorage,
   // |metadata_change_list| in case when some of the data changes are filtered
   // out, or even be empty in case when a commit confirmation is processed and
   // only the metadata needs to persisted.
-  syncer::ModelError ApplySyncChanges(
+  base::Optional<syncer::ModelError> ApplySyncChanges(
       std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
       syncer::EntityChangeList entity_changes) override;
 
@@ -148,7 +148,7 @@ class ReadingListStore : public ReadingListModelStorage,
       syncer::ModelTypeStore::Result result,
       std::unique_ptr<syncer::ModelTypeStore::RecordList> entries);
   void OnDatabaseSave(syncer::ModelTypeStore::Result result);
-  void OnReadAllMetadata(syncer::ModelError error,
+  void OnReadAllMetadata(base::Optional<syncer::ModelError> error,
                          std::unique_ptr<syncer::MetadataBatch> metadata_batch);
 
   void AddEntryToBatch(syncer::MutableDataBatch* batch,