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

chrome/browser/sync/glue/ui_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 "chrome/browser/sync/glue/ui_model_worker.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/message_loop/message_loop.h"
 #include "base/synchronization/waitable_event.h"
@@ skipping 29 matching lines @@
 }
 
 }  // namespace
 
 UIModelWorker::UIModelWorker(syncer::WorkerLoopDestructionObserver* observer)
     : syncer::ModelSafeWorker(observer) {
 }
 
 void UIModelWorker::RegisterForLoopDestruction() {
   CHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  base::MessageLoop::current()->AddDestructionObserver(this);
   SetWorkingLoopToCurrent();
 }
 
 syncer::SyncerError UIModelWorker::DoWorkAndWaitUntilDoneImpl(
     const syncer::WorkCallback& work) {
   syncer::SyncerError error_info;
   if (BrowserThread::CurrentlyOn(BrowserThread::UI)) {
     DLOG(WARNING) << "DoWorkAndWaitUntilDone called from "
       << "ui_loop_. Probably a nested invocation?";
     return work.Run();
@@ skipping 13 matching lines @@
 }
 
 syncer::ModelSafeGroup UIModelWorker::GetModelSafeGroup() {
   return syncer::GROUP_UI;
 }
 
 UIModelWorker::~UIModelWorker() {
 }
 
 }  // namespace browser_sync