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

chrome/browser/history/history_browsertest.cc

Index: chrome/browser/history/history_browsertest.cc
diff --git a/chrome/browser/history/history_browsertest.cc b/chrome/browser/history/history_browsertest.cc
index 49e813cc922f2001238e9d907b5b498c1842c1cb..fdf10ea3f9dd6138ab62e36d53d0e915c7470011 100644
--- a/chrome/browser/history/history_browsertest.cc
+++ b/chrome/browser/history/history_browsertest.cc
@@ -87,11 +87,11 @@ class HistoryBrowserTest : public InProcessBrowserTest {
 
   void WaitForHistoryBackendToRun() {
     base::CancelableTaskTracker task_tracker;
-    scoped_refptr<history::HistoryDBTask> task(new WaitForHistoryTask());
+    history::HistoryDBTask* task = new WaitForHistoryTask();
     HistoryService* history =
         HistoryServiceFactory::GetForProfile(GetProfile(),
                                              Profile::EXPLICIT_ACCESS);
-    history->HistoryService::ScheduleDBTask(task.get(), &task_tracker);
+    history->HistoryService::ScheduleDBTask(task, &task_tracker);
     content::RunMessageLoop();
   }