[Sync] Refactor ModelSafeWorker::DoWorkAndWaitUntilDone() to avoid code duplication.

components/sync/engine/passive_model_worker.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 "components/sync/engine/passive_model_worker.h"
 
-#include "base/callback.h"
-
 namespace syncer {
 
 PassiveModelWorker::PassiveModelWorker() = default;
 
 PassiveModelWorker::~PassiveModelWorker() {}
 
-SyncerError PassiveModelWorker::DoWorkAndWaitUntilDoneImpl(
-    const WorkCallback& work) {
-  // Simply do the work on the current thread.
-  return work.Run();
+void PassiveModelWorker::ScheduleWork(base::Closure work) {
+  work.Run();
 }
 
 ModelSafeGroup PassiveModelWorker::GetModelSafeGroup() {
   return GROUP_PASSIVE;
 }
 
 bool PassiveModelWorker::IsOnModelThread() {
   // Passive types are checked by SyncBackendRegistrar.
   NOTREACHED();
   return false;
 }
 
 }  // namespace syncer