Have content/ create and destroy its own threads.

chrome/browser/history/history.cc

 // Copyright (c) 2011 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.
 
 // The history system runs on a background thread so that potentially slow
 // database operations don't delay the browser. This backend processing is
 // represented by HistoryBackend. The HistoryService's job is to dispatch to
 // that thread.
 //
 // Main thread                       History thread
@@ skipping 12 matching lines @@
 //                                         (and favicons)
 
 #include "chrome/browser/history/history.h"
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/message_loop.h"
 #include "base/path_service.h"
 #include "base/string_util.h"
 #include "base/task.h"
+#include "base/threading/thread.h"
 #include "chrome/browser/autocomplete/history_url_provider.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/history/history_backend.h"
 #include "chrome/browser/history/history_notifications.h"
 #include "chrome/browser/history/history_types.h"
 #include "chrome/browser/history/in_memory_database.h"
 #include "chrome/browser/history/in_memory_history_backend.h"
 #include "chrome/browser/history/top_sites.h"
 #include "chrome/browser/prefs/pref_service.h"
 #include "chrome/browser/profiles/profile.h"
@@ skipping 801 matching lines @@
     history::TopSites* ts = profile_->GetTopSitesWithoutCreating();
     if (ts)
       ts->MigrateFromHistory();
   }
 }
 
 void HistoryService::OnTopSitesReady() {
   ScheduleAndForget(PRIORITY_NORMAL,
                     &HistoryBackend::MigrateThumbnailsDatabase);
 }