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

components/reading_list/ios/reading_list_store.cc

Index: components/reading_list/ios/reading_list_store.cc
diff --git a/components/reading_list/ios/reading_list_store.cc b/components/reading_list/ios/reading_list_store.cc
index 4be2da2ea22cfbe9f8a8d626e23a74c63f47af9c..528bcab51372967c450017b1bfc05b99a269c5b1 100644
--- a/components/reading_list/ios/reading_list_store.cc
+++ b/components/reading_list/ios/reading_list_store.cc
@@ -150,10 +150,10 @@ void ReadingListStore::OnDatabaseLoad(
 }
 
 void ReadingListStore::OnReadAllMetadata(
-    syncer::ModelError error,
+    base::Optional<syncer::ModelError> error,
     std::unique_ptr<syncer::MetadataBatch> metadata_batch) {
   DCHECK(CalledOnValidThread());
-  if (error.IsSet()) {
+  if (error) {
     change_processor()->ReportError(FROM_HERE, "Failed to read metadata.");
   } else {
     change_processor()->OnMetadataLoaded(std::move(metadata_batch));
@@ -198,7 +198,7 @@ ReadingListStore::CreateMetadataChangeList() {
 // 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 ReadingListStore::MergeSyncData(
+base::Optional<syncer::ModelError> ReadingListStore::MergeSyncData(
     std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
     syncer::EntityDataMap entity_data_map) {
   DCHECK(CalledOnValidThread());
@@ -277,7 +277,7 @@ syncer::ModelError ReadingListStore::MergeSyncData(
   }
   batch_->TransferMetadataChanges(std::move(metadata_change_list));
 
-  return syncer::ModelError();
+  return {};
 }
 
 // Apply changes from the sync server locally.
@@ -285,7 +285,7 @@ syncer::ModelError ReadingListStore::MergeSyncData(
 // |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 ReadingListStore::ApplySyncChanges(
+base::Optional<syncer::ModelError> ReadingListStore::ApplySyncChanges(
     std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
     syncer::EntityChangeList entity_changes) {
   DCHECK(CalledOnValidThread());
@@ -347,7 +347,7 @@ syncer::ModelError ReadingListStore::ApplySyncChanges(
   }
 
   batch_->TransferMetadataChanges(std::move(metadata_change_list));
-  return syncer::ModelError();
+  return {};
 }
 
 void ReadingListStore::GetData(StorageKeyList storage_keys,