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

chrome/browser/sync/glue/history_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/history_model_worker.h"
 
 #include "base/memory/ref_counted.h"
 #include "base/message_loop/message_loop.h"
 #include "base/synchronization/waitable_event.h"
 #include "content/public/browser/browser_thread.h"
@@ skipping 82 matching lines @@
 
 void HistoryModelWorker::RegisterForLoopDestruction() {
   CHECK(history_service_.get());
   history_service_->ScheduleDBTask(
       scoped_ptr<history::HistoryDBTask>(new AddDBThreadObserverTask(
           base::Bind(&HistoryModelWorker::RegisterOnDBThread, this))),
       cancelable_tracker_.get());
 }
 
 void HistoryModelWorker::RegisterOnDBThread() {
+  base::MessageLoop::current()->AddDestructionObserver(this);
   SetWorkingLoopToCurrent();
 }
 
 syncer::SyncerError HistoryModelWorker::DoWorkAndWaitUntilDoneImpl(
     const syncer::WorkCallback& work) {
   syncer::SyncerError error = syncer::UNSET;
   if (BrowserThread::PostTask(BrowserThread::UI,
                               FROM_HERE,
                               base::Bind(&PostWorkerTask,
                                          history_service_,
@@ skipping 13 matching lines @@
 }
 
 HistoryModelWorker::~HistoryModelWorker() {
   // The base::CancelableTaskTracker class is not thread-safe and must only be
   // used from a single thread but the current object may not be destroyed from
   // the UI thread, so delete it from the UI thread.
   BrowserThread::DeleteOnUIThread::Destruct(cancelable_tracker_.release());
 }
 
 }  // namespace browser_sync