Make HistoryDBTask not refcounted, and ensure it's destroyed on its origin thread.

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 53 matching lines @@
 
 // Post the work task on |history_service|'s DB thread from the UI
 // thread.
 void PostWorkerTask(const base::WeakPtr<HistoryService>& history_service,
                     const syncer::WorkCallback& work,
                     base::CancelableTaskTracker* cancelable_tracker,
                     WaitableEvent* done,
                     syncer::SyncerError* error) {
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
   if (history_service.get()) {
-    scoped_refptr<WorkerTask> task(new WorkerTask(work, done, error));
-    history_service->ScheduleDBTask(task.get(), cancelable_tracker);
+    WorkerTask* task = new WorkerTask(work, done, error);
+    history_service->ScheduleDBTask(task, cancelable_tracker);
   } else {
     *error = syncer::CANNOT_DO_WORK;
     done->Signal();
   }
 }
 
 }  // namespace
 
 HistoryModelWorker::HistoryModelWorker(
     const base::WeakPtr<HistoryService>& history_service,
@@ skipping 41 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