Do not observe MessageLoop destruction from ModelSafeWorker.

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/message_loop/message_loop.h"
+#include "base/callback.h"
 
 namespace syncer {
 
-PassiveModelWorker::PassiveModelWorker(WorkerLoopDestructionObserver* observer)
-    : ModelSafeWorker(observer) {}
+PassiveModelWorker::PassiveModelWorker() = default;
 
 PassiveModelWorker::~PassiveModelWorker() {}
 
-void PassiveModelWorker::RegisterForLoopDestruction() {
-  SetWorkingLoopToCurrent();
-}
-
 SyncerError PassiveModelWorker::DoWorkAndWaitUntilDoneImpl(
     const WorkCallback& work) {
   // Simply do the work on the current thread.
   return work.Run();
 }
 
 ModelSafeGroup PassiveModelWorker::GetModelSafeGroup() {
   return GROUP_PASSIVE;
 }
 
 }  // namespace syncer