Add missing IWYU message_loop.h includes.

chrome/browser/extensions/api/socket/udp_socket_unittest.cc

 // Copyright (c) 2012 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 <stddef.h>
 #include <stdint.h>
 
 #include <memory>
 #include <string>
 
 #include "base/location.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/test/test_timeouts.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chrome/test/base/browser_with_test_window_test.h"
 #include "extensions/browser/api/socket/udp_socket.h"
 #include "net/base/io_buffer.h"
 #include "net/base/ip_address.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 125 matching lines @@
   // If not received within the test action timeout, quit the message loop.
   io_loop.task_runner()->PostDelayedTask(FROM_HERE, run_loop.QuitClosure(),
                                          TestTimeouts::action_timeout());
 
   run_loop.Run();
 
   EXPECT_TRUE(packet_received) << "Failed to receive from multicast address";
 }
 
 }  // namespace extensions