Do not observe MessageLoop destruction from ModelSafeWorker.

components/sync/tools/sync_client.cc

 // Copyright (c) 2012 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 <stdint.h>
 
 #include <cstddef>
 #include <cstdio>
 #include <memory>
 #include <string>
@@ skipping 365 matching lines @@
   model_types.Put(PRIORITY_PREFERENCES);
   model_types.Put(DICTIONARY);
   model_types.Put(FAVICON_IMAGES);
   model_types.Put(FAVICON_TRACKING);
 
   ModelSafeRoutingInfo routing_info;
   for (ModelTypeSet::Iterator it = model_types.First(); it.Good(); it.Inc()) {
     routing_info[it.Get()] = GROUP_PASSIVE;
   }
   scoped_refptr<PassiveModelWorker> passive_model_safe_worker =
-      new PassiveModelWorker(nullptr);
+      new PassiveModelWorker();
   std::vector<scoped_refptr<ModelSafeWorker>> workers;
   workers.push_back(passive_model_safe_worker);
 
   // Set up sync manager.
   SyncManagerFactory sync_manager_factory;
   std::unique_ptr<SyncManager> sync_manager =
       sync_manager_factory.CreateSyncManager("sync_client manager");
   LoggingJsEventHandler js_event_handler;
   // Used only by InitialProcessMetadata(), so it's okay to leave this as null.
   const scoped_refptr<base::TaskRunner> blocking_task_runner = nullptr;
@@ skipping 52 matching lines @@
   io_thread.Stop();
   return 0;
 }
 
 }  // namespace
 }  // namespace syncer
 
 int main(int argc, char* argv[]) {
   return syncer::SyncClientMain(argc, argv);
 }