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

chrome/browser/history/in_memory_url_index.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/in_memory_url_index.h"
 
 #include "base/debug/trace_event.h"
 #include "base/file_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "chrome/browser/bookmarks/bookmark_model_factory.h"
@@ skipping 278 matching lines @@
   }
 }
 
 // Restoring from the History DB -----------------------------------------------
 
 void InMemoryURLIndex::ScheduleRebuildFromHistory() {
   HistoryService* service =
       HistoryServiceFactory::GetForProfile(profile_,
                                            Profile::EXPLICIT_ACCESS);
   service->ScheduleDBTask(
-      new InMemoryURLIndex::RebuildPrivateDataFromHistoryDBTask(
-          this, languages_, scheme_whitelist_),
+      scoped_ptr<history::HistoryDBTask>(
+          new InMemoryURLIndex::RebuildPrivateDataFromHistoryDBTask(
+              this, languages_, scheme_whitelist_)),
       &cache_reader_tracker_);
 }
 
 void InMemoryURLIndex::DoneRebuidingPrivateDataFromHistoryDB(
     bool succeeded,
     scoped_refptr<URLIndexPrivateData> private_data) {
   DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
   if (succeeded) {
     private_data_tracker_.TryCancelAll();
     private_data_ = private_data;
@@ skipping 40 matching lines @@
         base::Bind(DeleteCacheFile, path));
   }
 }
 
 void InMemoryURLIndex::OnCacheSaveDone(bool succeeded) {
   if (save_cache_observer_)
     save_cache_observer_->OnCacheSaveFinished(succeeded);
 }
 
 }  // namespace history