Add missing IWYU message_loop.h includes.

components/update_client/crx_downloader_unittest.cc

 // Copyright 2013 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 "components/update_client/crx_downloader.h"
 
 #include <memory>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/memory/ref_counted.h"
+#include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "components/update_client/update_client_errors.h"
 #include "net/base/net_errors.h"
 #include "net/url_request/test_url_request_interceptor.h"
 #include "net/url_request/url_request_test_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 404 matching lines @@
 
   EXPECT_EQ(0, get_interceptor_->GetHitCount());
 
   EXPECT_EQ(1, num_download_complete_calls_);
   EXPECT_EQ(kExpectedContext, crx_context_);
   EXPECT_NE(0, download_complete_result_.error);
   EXPECT_TRUE(download_complete_result_.response.empty());
 }
 
 }  // namespace update_client