Have content/ create and destroy its own threads.

content/content_browser.gypi

 # 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.
 
 {
   'dependencies': [
     '../crypto/crypto.gyp:crypto',
     '../net/net.gyp:net',
     '../net/net.gyp:http_server',
     '../ppapi/ppapi_internal.gyp:ppapi_proxy',
@@ skipping 39 matching lines @@
     'browser/appcache/chrome_appcache_service.h',
     'browser/browser_child_process_host.cc',
     'browser/browser_child_process_host.h',
     'browser/browser_context.h',
     'browser/browser_main.cc',
     'browser/browser_main.h',
     'browser/browser_main_loop.cc',
     'browser/browser_main_loop.h',
     'browser/browser_message_filter.cc',
     'browser/browser_message_filter.h',
+    'browser/browser_process_io_thread.cc',
+    'browser/browser_process_io_thread.h',
     'browser/browser_process_sub_thread.cc',
     'browser/browser_process_sub_thread.h',
     'browser/browser_thread_impl.cc',
     'browser/browser_thread_impl.h',
     'browser/browser_url_handler.cc',
     'browser/browser_url_handler.h',
     'browser/browsing_instance.cc',
     'browser/browsing_instance.h',
     'browser/cancelable_request.cc',
     'browser/cancelable_request.h',
@@ skipping 504 matching lines @@
     'browser/worker_host/worker_service.cc',
     'browser/worker_host/worker_service.h',
     'browser/worker_host/worker_service_observer.h',
     'browser/zygote_host_linux.cc',
     'browser/zygote_host_linux.h',
     'browser/zygote_main_linux.cc',
     'public/browser/browser_main_parts.h',
     'public/browser/browser_thread.h',
     'public/browser/content_browser_client.h',
     'public/browser/download_manager_delegate.h',
+    'public/browser/io_thread_delegate.h',
     'public/browser/native_web_keyboard_event.h',
     'public/browser/navigation_types.h',
     'public/browser/notification_details.h',
     'public/browser/notification_observer.h',
     'public/browser/notification_registrar.cc',
     'public/browser/notification_registrar.h',
     'public/browser/notification_service.h',
     'public/browser/notification_source.h',
     'public/browser/notification_types.h',
     'public/browser/resource_dispatcher_host_delegate.h',
@@ skipping 178 matching lines @@
         'browser/renderer_host/java/java_bridge_channel_host.h',
         'browser/renderer_host/java/java_bridge_dispatcher_host_manager.cc',
         'browser/renderer_host/java/java_bridge_dispatcher_host_manager.h',
         'browser/renderer_host/java/java_bridge_dispatcher_host.cc',
         'browser/renderer_host/java/java_bridge_dispatcher_host.h',
       ],
     }],
   ],
 }