Add missing IWYU message_loop.h includes.

content/public/test/browser_test_base.cc

 // Copyright (c) 2011 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 "content/public/test/browser_test_base.h"
 
 #include <stddef.h>
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/debug/stack_trace.h"
 #include "base/feature_list.h"
 #include "base/i18n/icu_util.h"
 #include "base/location.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/sys_info.h"
 #include "base/test/test_timeouts.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "base/threading/thread_restrictions.h"
 #include "build/build_config.h"
 #include "content/browser/renderer_host/render_process_host_impl.h"
 #include "content/browser/tracing/tracing_controller_impl.h"
 #include "content/public/app/content_main.h"
@@ skipping 381 matching lines @@
   if (mojo_rules.empty())
     return;
 
   mojom::NetworkServiceTestPtr network_service_test;
   ServiceManagerConnection::GetForProcess()->GetConnector()->BindInterface(
       mojom::kNetworkServiceName, &network_service_test);
   network_service_test->AddRules(std::move(mojo_rules));
 }
 
 }  // namespace content