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

chrome/browser/prerender/prerender_local_predictor.cc

Index: chrome/browser/prerender/prerender_local_predictor.cc
diff --git a/chrome/browser/prerender/prerender_local_predictor.cc b/chrome/browser/prerender/prerender_local_predictor.cc
index 2e451e94c0fb0563987931cb9981a9def3b2db0b..0b686095936e49d53fd1195ee7920f574668e114 100644
--- a/chrome/browser/prerender/prerender_local_predictor.cc
+++ b/chrome/browser/prerender/prerender_local_predictor.cc
@@ -555,10 +555,12 @@ void PrerenderLocalPredictor::OnAddVisit(const history::BriefVisitInfo& info) {
     RecordEvent(EVENT_GOT_HISTORY_ISSUING_LOOKUP);
     CandidatePrerenderInfo* lookup_info_ptr = lookup_info.get();
     history->ScheduleDBTask(
-        new GetURLForURLIDTask(lookup_info_ptr,
-                               base::Bind(&PrerenderLocalPredictor::OnLookupURL,
-                                          base::Unretained(this),
-                                          base::Passed(&lookup_info))),
+        scoped_ptr<history::HistoryDBTask>(
+            new GetURLForURLIDTask(
+                lookup_info_ptr,
+                base::Bind(&PrerenderLocalPredictor::OnLookupURL,
+                           base::Unretained(this),
+                           base::Passed(&lookup_info)))),
         &history_db_tracker_);
   }
 }
@@ -998,8 +1000,10 @@ void PrerenderLocalPredictor::Init() {
   HistoryService* history = GetHistoryIfExists();
   if (history) {
     CHECK(!is_visit_database_observer_);
-    history->ScheduleDBTask(new GetVisitHistoryTask(this, kMaxVisitHistory),
-                            &history_db_tracker_);
+    history->ScheduleDBTask(
+        scoped_ptr<history::HistoryDBTask>(
+            new GetVisitHistoryTask(this, kMaxVisitHistory)),
+        &history_db_tracker_);
     history->AddVisitDatabaseObserver(this);
     is_visit_database_observer_ = true;
   } else {