Revert "Sync: Avoid deadlock in SyncBackendRegistrar / ModelSafeWorker on sync backend shutdown."

sync/internal_api/public/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 "sync/internal_api/public/engine/passive_model_worker.h"
 
 #include "base/message_loop/message_loop.h"
 
 namespace syncer {
 
 PassiveModelWorker::PassiveModelWorker(const base::MessageLoop* sync_loop,
                                        WorkerLoopDestructionObserver* observer)
     : ModelSafeWorker(observer),
       sync_loop_(sync_loop) {
 }
 
 PassiveModelWorker::~PassiveModelWorker() {
 }
 
 void PassiveModelWorker::RegisterForLoopDestruction() {
+  base::MessageLoop::current()->AddDestructionObserver(this);
   SetWorkingLoopToCurrent();
 }
 
 SyncerError PassiveModelWorker::DoWorkAndWaitUntilDoneImpl(
     const WorkCallback& work) {
   DCHECK_EQ(base::MessageLoop::current(), sync_loop_);
   // Simply do the work on the current thread.
   return work.Run();
 }
 
 ModelSafeGroup PassiveModelWorker::GetModelSafeGroup() {
   return GROUP_PASSIVE;
 }
 
 }  // namespace syncer