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

chrome/browser/sync/test/integration/two_client_uss_sync_test.cc

 // 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 "base/macros.h"
 #include "base/memory/ptr_util.h"
+#include "base/optional.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chrome/browser/sync/chrome_sync_client.h"
 #include "chrome/browser/sync/profile_sync_service_factory.h"
 #include "chrome/browser/sync/test/integration/profile_sync_service_harness.h"
 #include "chrome/browser/sync/test/integration/single_client_status_change_checker.h"
 #include "chrome/browser/sync/test/integration/status_change_checker.h"
 #include "chrome/browser/sync/test/integration/sync_integration_test_util.h"
 #include "chrome/browser/sync/test/integration/sync_test.h"
 #include "components/browser_sync/profile_sync_components_factory_impl.h"
 #include "components/browser_sync/profile_sync_service.h"
@@ skipping 43 matching lines @@
   class Observer {
    public:
     virtual void OnApplySyncChanges() = 0;
   };
 
   TestModelTypeSyncBridge()
       : FakeModelTypeSyncBridge(base::Bind(&ModelTypeChangeProcessor::Create)) {
     change_processor()->OnMetadataLoaded(db().CreateMetadataBatch());
   }
 
-  syncer::ModelError ApplySyncChanges(
+  base::Optional<syncer::ModelError> ApplySyncChanges(
       std::unique_ptr<syncer::MetadataChangeList> metadata_changes,
       syncer::EntityChangeList entity_changes) override {
-    syncer::ModelError error = FakeModelTypeSyncBridge::ApplySyncChanges(
+    auto error = FakeModelTypeSyncBridge::ApplySyncChanges(
         std::move(metadata_changes), entity_changes);
     NotifyObservers();
     return error;
   }
 
   void AddObserver(Observer* observer) { observers_.insert(observer); }
   void RemoveObserver(Observer* observer) { observers_.erase(observer); }
 
  private:
   void NotifyObservers() {
@@ skipping 277 matching lines @@
 
   ASSERT_TRUE(DataChecker(model1, kKey1, kValue2).Wait());
   ASSERT_TRUE(DataChecker(model1, kKey2, kValue1).Wait());
   ASSERT_TRUE(DataChecker(model1, kKey4, kValue1).Wait());
 
   ASSERT_TRUE(DataChecker(model0, kKey1, kValue2).Wait());
   ASSERT_TRUE(DataChecker(model0, kKey3, kValue1).Wait());
 }
 
 }  // namespace