Add missing IWYU message_loop.h includes.

components/dom_distiller/standalone/content_extractor_browsertest.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 <stddef.h>
 #include <sstream>
 #include <utility>
 
 #include "base/command_line.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/id_map.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/dom_distiller/content/browser/distiller_javascript_utils.h"
 #include "components/dom_distiller/content/browser/distiller_page_web_contents.h"
 #include "components/dom_distiller/core/article_entry.h"
 #include "components/dom_distiller/core/distilled_page_prefs.h"
@@ skipping 417 matching lines @@
       protobuf_output_stream_;
 };
 
 IN_PROC_BROWSER_TEST_F(ContentExtractor, MANUAL_ExtractUrl) {
   SetDistillerJavaScriptWorldId(content::ISOLATED_WORLD_ID_CONTENT_END);
   Start();
   base::RunLoop().Run();
 }
 
 }  // namespace dom_distiller