Add missing IWYU message_loop.h includes.

chrome/browser/chromeos/login/saml/saml_offline_signin_limiter_unittest.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 "chrome/browser/chromeos/login/saml/saml_offline_signin_limiter.h"
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/message_loop/message_loop.h"
 #include "base/test/simple_test_clock.h"
 #include "base/test/test_simple_task_runner.h"
 #include "base/time/clock.h"
 #include "chrome/browser/chromeos/login/saml/saml_offline_signin_limiter_factory.h"
 #include "chrome/browser/chromeos/login/users/mock_user_manager.h"
 #include "chrome/browser/chromeos/login/users/scoped_user_manager_enabler.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/common/pref_names.h"
 #include "chrome/test/base/testing_browser_process.h"
 #include "chrome/test/base/testing_profile.h"
@@ skipping 633 matching lines @@
   EXPECT_CALL(*user_manager_, SaveForceOnlineSignin(test_account_id_, true))
       .Times(1);
   limiter_->SignedIn(UserContext::AUTH_FLOW_OFFLINE);
 
   const base::Time last_gaia_signin_time = base::Time::FromInternalValue(
       prefs->GetInt64(prefs::kSAMLLastGAIASignInTime));
   EXPECT_EQ(gaia_signin_time, last_gaia_signin_time);
 }
 
 } //  namespace chromeos