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

components/reading_list/ios/reading_list_model_unittest.mm

 // 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.
 
 #include "components/reading_list/ios/reading_list_model.h"
 
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #import "base/test/ios/wait_util.h"
 #include "components/reading_list/ios/reading_list_model_impl.h"
@@ skipping 92 matching lines @@
     return std::unique_ptr<ScopedBatchUpdate>();
   }
 
   // Syncing is not used in this test class.
   std::unique_ptr<syncer::MetadataChangeList> CreateMetadataChangeList()
       override {
     NOTREACHED();
     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 {
     NOTREACHED();
     return;
   }
 
   void GetAllData(DataCallback callback) override {
     NOTREACHED();
     return;
@@ skipping 502 matching lines @@
 }
 
 // Tests that ReadingListModel calls CallbackModelBeingDeleted when destroyed.
 TEST_F(ReadingListModelTest, CallbackModelBeingDeleted) {
   AssertObserverCount(1, 0, 0, 0, 0, 0, 0, 0, 0);
   model_.reset();
   AssertObserverCount(1, 0, 0, 1, 0, 0, 0, 0, 0);
 }
 
 }  // namespace