Add missing IWYU message_loop.h includes.

chrome/browser/media/webrtc/tab_desktop_media_list_unittest.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 "chrome/browser/media/webrtc/tab_desktop_media_list.h"
 
 #include "base/command_line.h"
 #include "base/files/file_util.h"
 #include "base/location.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chrome/browser/media/webrtc/desktop_media_list_observer.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/browser/ui/browser_list.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/test/base/scoped_testing_local_state.h"
 #include "chrome/test/base/test_browser_window.h"
@@ skipping 330 matching lines @@
   favicon_info.image = CreateGrayscaleImage(gfx::Size(10, 10), 100);
   contents->GetController().GetTransientEntry()->GetFavicon() = favicon_info;
 
   EXPECT_CALL(observer_, OnSourceThumbnailChanged(list_.get(), 0))
       .WillOnce(QuitMessageLoop());
 
   base::RunLoop().Run();
 
   list_.reset();
 }