Add missing IWYU message_loop.h includes.

headless/public/util/deterministic_dispatcher_test.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 "headless/public/util/deterministic_dispatcher.h"
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "headless/public/util/navigation_request.h"
 #include "headless/public/util/testing/fake_managed_dispatch_url_request_job.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 using testing::ElementsAre;
 
 namespace headless {
@@ skipping 166 matching lines @@
   base::RunLoop().RunUntilIdle();
   EXPECT_THAT(notifications, ElementsAre("id: 1 OK", "id: 2 OK", "id: 3 OK"));
 
   job3.reset();
   base::RunLoop().RunUntilIdle();
   EXPECT_THAT(notifications,
               ElementsAre("id: 1 OK", "id: 2 OK", "id: 3 OK", "id: 4 OK"));
 }
 
 }  // namespace headless