Add missing IWYU message_loop.h includes.

chrome/browser/ui/desktop_ios_promotion/sms_service_unittest.cc

 // Copyright 2017 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 "chrome/browser/ui/desktop_ios_promotion/sms_service.h"
 
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "components/signin/core/browser/account_tracker_service.h"
 #include "components/signin/core/browser/fake_profile_oauth2_token_service.h"
 #include "components/signin/core/browser/fake_signin_manager.h"
 #include "components/signin/core/browser/test_signin_client.h"
 #include "net/http/http_status_code.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace {
@@ skipping 173 matching lines @@
       base::Bind(&TestingSMSService::QueryPhoneNumberCallbackFailed,
                  base::Unretained(sms_service())));
   base::RunLoop().RunUntilIdle();
   std::string send_sms_invalid = "{}";
   sms_service()->SetNextRequestResponseData(net::HTTP_OK, send_sms_invalid);
   sms_service()->SendSMS(
       promo_id, base::Bind(&TestingSMSService::QueryPhoneNumberCallbackFailed,
                            base::Unretained(sms_service())));
   base::RunLoop().RunUntilIdle();
 }