Add missing IWYU message_loop.h includes.

ppapi/nacl_irt/plugin_startup.cc

 // Copyright 2014 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 "ppapi/nacl_irt/plugin_startup.h"
 
 #include "base/bind.h"
 #include "base/file_descriptor_posix.h"
 #include "base/location.h"
 #include "base/logging.h"
+#include "base/message_loop/message_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/thread.h"
 #include "ipc/ipc_channel_handle.h"
 #include "ppapi/nacl_irt/manifest_service.h"
 #include "ppapi/shared_impl/ppb_audio_shared.h"
 
 namespace ppapi {
 namespace {
 
@@ skipping 101 matching lines @@
   // The IOThread must be initialized in advance.
   DCHECK(g_io_thread);
   return g_io_thread;
 }
 
 ManifestService* GetManifestService() {
   return g_manifest_service;
 }
 
 }  // namespace ppapi