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

chrome/browser/history/top_sites_impl_unittest.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 "base/bind.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/task/cancelable_task_tracker.h"
@@ skipping 163 matching lines @@
   // loaded from the db.
   void RefreshTopSitesAndRecreate() {
     StartQueryForMostVisited();
     WaitForHistory();
     RecreateTopSitesAndBlock();
   }
 
   // Blocks the caller until history processes a task. This is useful if you
   // need to wait until you know history has processed a task.
   void WaitForHistory() {
-    history_service()->ScheduleDBTask(new WaitForHistoryTask(),
-                                      &history_tracker_);
+    history_service()->ScheduleDBTask(
+        scoped_ptr<history::HistoryDBTask>(new WaitForHistoryTask()),
+        &history_tracker_);
     base::MessageLoop::current()->Run();
   }
 
   // Waits for top sites to finish processing a task. This is useful if you need
   // to wait until top sites finishes processing a task.
   void WaitForTopSites() {
     top_sites()->backend_->DoEmptyRequest(
         base::Bind(&TopSitesImplTest::QuitCallback, base::Unretained(this)),
         &top_sites_tracker_);
     base::MessageLoop::current()->Run();
@@ skipping 1370 matching lines @@
   ASSERT_EQ(orig_thumbnail.getSize(), thumbnail.getSize());
   orig_thumbnail.lockPixels();
   thumbnail.lockPixels();
   EXPECT_EQ(0, memcmp(orig_thumbnail.getPixels(), thumbnail.getPixels(),
                       orig_thumbnail.getSize()));
   thumbnail.unlockPixels();
   orig_thumbnail.unlockPixels();
 }
 
 }  // namespace history