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

chrome/browser/history/url_index_private_data.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/history/url_index_private_data.h"
 
 #include <functional>
 #include <iterator>
 #include <limits>
 #include <numeric>
@@ skipping 348 matching lines @@
   // Ignore this update.  This can happen if, for instance, the user
   // removes the URL from URLIndexPrivateData before the historyDB call
   // returns.
 }
 
 void URLIndexPrivateData::ScheduleUpdateRecentVisits(
     HistoryService* history_service,
     URLID url_id,
     base::CancelableTaskTracker* tracker) {
   history_service->ScheduleDBTask(
-      new UpdateRecentVisitsFromHistoryDBTask(this, url_id), tracker);
+      scoped_ptr<history::HistoryDBTask>(
+          new UpdateRecentVisitsFromHistoryDBTask(this, url_id)), tracker);
 }
 
 // Helper functor for DeleteURL.
 class HistoryInfoMapItemHasURL {
  public:
   explicit HistoryInfoMapItemHasURL(const GURL& url): url_(url) {}
 
   bool operator()(const std::pair<const HistoryID, HistoryInfoMapValue>& item) {
     return item.second.url_row.url() == url_;
   }
@@ skipping 960 matching lines @@
   // recently visited (within the last 12/24 hours) as highly important. Get
   // input from mpearson.
   if (r1.typed_count() != r2.typed_count())
     return (r1.typed_count() > r2.typed_count());
   if (r1.visit_count() != r2.visit_count())
     return (r1.visit_count() > r2.visit_count());
   return (r1.last_visit() > r2.last_visit());
 }
 
 }  // namespace history