Add missing IWYU message_loop.h includes.

chrome/utility/image_writer/image_writer_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 <stdint.h>
 #include <string.h>
 
 #include <string>
 
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "chrome/utility/image_writer/error_messages.h"
 #include "chrome/utility/image_writer/image_writer.h"
 #include "chrome/utility/image_writer/image_writer_handler.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace image_writer {
 
 using testing::_;
@@ skipping 203 matching lines @@
   EXPECT_CALL(mock_handler, SendFailed(_)).Times(0);
 
   FillDefault(image_path_);
 
   image_writer.Write();
 
   base::RunLoop().RunUntilIdle();
 }
 
 }  // namespace image_writer