Add missing IWYU message_loop.h includes.

components/bookmarks/test/bookmark_test_helpers.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 "components/bookmarks/test/bookmark_test_helpers.h"
 
 #include <stddef.h>
 
 #include "base/callback.h"
 #include "base/logging.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/utf_string_conversions.h"
 #include "components/bookmarks/browser/base_bookmark_model_observer.h"
 #include "components/bookmarks/browser/bookmark_model.h"
 #include "url/gurl.h"
 
 namespace bookmarks {
 namespace test {
 
 namespace {
@@ skipping 110 matching lines @@
                              const std::string& model_string) {
   DCHECK(node);
   std::string::size_type start_pos = 0;
   std::string::size_type end_pos =
       AddNodesFromString(model, node, model_string, start_pos);
   DCHECK(end_pos == std::string::npos);
 }
 
 }  // namespace test
 }  // namespace bookmarks