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

components/reading_list/ios/reading_list_store.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_READING_LIST_IOS_READING_LIST_STORE_H_
 #define COMPONENTS_READING_LIST_IOS_READING_LIST_STORE_H_
 
 #include "base/threading/non_thread_safe.h"
 #include "components/reading_list/ios/reading_list_model_storage.h"
 #include "components/reading_list/ios/reading_list_store_delegate.h"
@@ skipping 37 matching lines @@
   // keys in the |entity_data_map| will have been created via GetClientTag(...),
   // and if a local and sync data should match/merge but disagree on tags, the
   // service should use the sync data's tag. Any local pieces of data that are
   // not present in sync should immediately be Put(...) to the processor before
   // returning. The same MetadataChangeList that was passed into this function
   // can be passed to Put(...) calls. Delete(...) can also be called but should
   // not be needed for most model types. Durable storage writes, if not able to
   // 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;
 
   // Apply changes from the sync server locally.
   // Please note that |entity_changes| might have fewer entries than
   // |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;
 
   // Returns whether entries respect a strict order for sync and if |rhs| can be
   // submitted to sync after |lhs| has been received.
   // The order should ensure that there is no sync loop in sync and should be
   // submitted to sync in strictly increasing order.
   // Entries are in increasing order if all the fields respect increasing order.
   // - URL must be the same.
   // - update_title_time_us:
@@ skipping 63 matching lines @@
   };
 
  private:
   void BeginTransaction();
   void CommitTransaction();
   // Callbacks needed for the database handling.
   void OnDatabaseLoad(
       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,
                        const ReadingListEntry& entry);
 
   std::unique_ptr<syncer::ModelTypeStore> store_;
   ReadingListModel* model_;
   ReadingListStoreDelegate* delegate_;
   StoreFactoryFunction create_store_callback_;
 
   int pending_transaction_count_;
   std::unique_ptr<syncer::ModelTypeStore::WriteBatch> batch_;
 };
 
 #endif  // COMPONENTS_READING_LIST_IOS_READING_LIST_STORE_H_