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

chrome/browser/sync/glue/browser_thread_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/browser_thread_model_worker.h"
 
 #include "base/bind.h"
 #include "base/synchronization/waitable_event.h"
 #include "content/public/browser/browser_thread.h"
 
@@ skipping 32 matching lines @@
 }
 
 syncer::ModelSafeGroup BrowserThreadModelWorker::GetModelSafeGroup() {
   return group_;
 }
 
 BrowserThreadModelWorker::~BrowserThreadModelWorker() {}
 
 void BrowserThreadModelWorker::RegisterForLoopDestruction() {
   if (BrowserThread::CurrentlyOn(thread_)) {
+    base::MessageLoop::current()->AddDestructionObserver(this);
     SetWorkingLoopToCurrent();
   } else {
     BrowserThread::PostTask(
         thread_, FROM_HERE,
         Bind(&BrowserThreadModelWorker::RegisterForLoopDestruction, this));
   }
 }
 
 void BrowserThreadModelWorker::CallDoWorkAndSignalTask(
     const syncer::WorkCallback& work,
@@ skipping 28 matching lines @@
 void FileModelWorker::CallDoWorkAndSignalTask(
     const syncer::WorkCallback& work,
     WaitableEvent* done,
     syncer::SyncerError* error) {
   BrowserThreadModelWorker::CallDoWorkAndSignalTask(work, done, error);
 }
 
 FileModelWorker::~FileModelWorker() {}
 
 }  // namespace browser_sync