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

chrome/browser/history/history_service.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.
 
 // The history system runs on a background thread so that potentially slow
 // database operations don't delay the browser. This backend processing is
 // represented by HistoryBackend. The HistoryService's job is to dispatch to
 // that thread.
 //
 // Main thread                       History thread
@@ skipping 360 matching lines @@
   ScheduleAndForget(PRIORITY_UI, &HistoryBackend::DeleteMatchingURLsForKeyword,
                     keyword_id, term);
 }
 
 void HistoryService::URLsNoLongerBookmarked(const std::set<GURL>& urls) {
   DCHECK(thread_checker_.CalledOnValidThread());
   ScheduleAndForget(PRIORITY_NORMAL, &HistoryBackend::URLsNoLongerBookmarked,
                     urls);
 }
 
-void HistoryService::ScheduleDBTask(scoped_refptr<history::HistoryDBTask> task,
+void HistoryService::ScheduleDBTask(scoped_ptr<history::HistoryDBTask> task,
                                     base::CancelableTaskTracker* tracker) {
   DCHECK(thread_checker_.CalledOnValidThread());
   base::CancelableTaskTracker::IsCanceledCallback is_canceled;
   tracker->NewTrackedTaskId(&is_canceled);
   // Use base::ThreadTaskRunnerHandler::Get() to get a message loop proxy to
   // the current message loop so that we can forward the call to the method
   // HistoryDBTask::DoneRunOnMainThread in the correct thread.
   thread_->message_loop_proxy()->PostTask(
       FROM_HERE,
       base::Bind(&HistoryBackend::ProcessDBTask,
                  history_backend_.get(),
-                 task,
+                 base::Passed(&task),
                  base::ThreadTaskRunnerHandle::Get(),
                  is_canceled));
 }
 
 void HistoryService::FlushForTest(const base::Closure& flushed) {
   thread_->message_loop_proxy()->PostTaskAndReply(
       FROM_HERE, base::Bind(&base::DoNothing), flushed);
 }
 
 void HistoryService::SetOnBackendDestroyTask(const base::Closure& task) {
@@ skipping 790 matching lines @@
   DCHECK(thread_checker_.CalledOnValidThread());
   visit_database_observers_.RemoveObserver(observer);
 }
 
 void HistoryService::NotifyVisitDBObserversOnAddVisit(
     const history::BriefVisitInfo& info) {
   DCHECK(thread_checker_.CalledOnValidThread());
   FOR_EACH_OBSERVER(history::VisitDatabaseObserver, visit_database_observers_,
                     OnAddVisit(info));
 }