Add missing IWYU message_loop.h includes.

services/video_capture/test/mock_device_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 "services/video_capture/test/mock_device_test.h"
 
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "media/capture/video/video_capture_jpeg_decoder.h"
 #include "media/capture/video/video_capture_system_impl.h"
 
 using testing::_;
 using testing::Invoke;
 using testing::InvokeWithoutArgs;
 
 namespace {
 
@@ skipping 90 matching lines @@
   requested_settings_.resolution_change_policy =
       media::RESOLUTION_POLICY_FIXED_RESOLUTION;
   requested_settings_.power_line_frequency =
       media::PowerLineFrequency::FREQUENCY_DEFAULT;
 
   mock_receiver_ =
       base::MakeUnique<MockReceiver>(mojo::MakeRequest(&mock_receiver_proxy_));
 }
 
 }  // namespace video_capture