Add missing IWYU message_loop.h includes.

chrome/browser/chromeos/arc/intent_helper/arc_settings_service_browsertest.cc

 // Copyright 2016 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 <memory>
 #include <string>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/command_line.h"
 #include "base/json/json_reader.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/values.h"
 #include "chrome/browser/chromeos/arc/intent_helper/arc_settings_service.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/common/pref_names.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chromeos/dbus/dbus_thread_manager.h"
 #include "chromeos/dbus/shill_profile_client.h"
 #include "chromeos/dbus/shill_service_client.h"
@@ skipping 646 matching lines @@
                                    ProxyPrefs::kFixedServersProxyModeName);
   expected_proxy_config->SetString("host", "proxy-n300");
   expected_proxy_config->SetInteger("port", 3000);
 
   EXPECT_EQ(CountProxyBroadcasts(fake_intent_helper_instance_->broadcasts(),
                                  expected_proxy_config.get()),
             1);
 }
 
 }  // namespace arc