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

chrome/browser/history/history_service.cc

Index: chrome/browser/history/history_service.cc
diff --git a/chrome/browser/history/history_service.cc b/chrome/browser/history/history_service.cc
index fe4aab27958fd1865b8390e0818cceef6be8b0ba..cf2bb5b13bba119cb8cf64358cc9994c2a9fedd3 100644
--- a/chrome/browser/history/history_service.cc
+++ b/chrome/browser/history/history_service.cc
@@ -378,7 +378,7 @@ void HistoryService::URLsNoLongerBookmarked(const std::set<GURL>& urls) {
                     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;
@@ -390,7 +390,7 @@ void HistoryService::ScheduleDBTask(scoped_refptr<history::HistoryDBTask> task,
       FROM_HERE,
       base::Bind(&HistoryBackend::ProcessDBTask,
                  history_backend_.get(),
-                 task,
+                 base::Passed(&task),
                  base::ThreadTaskRunnerHandle::Get(),
                  is_canceled));
 }