Add missing IWYU message_loop.h includes.

components/update_client/request_sender_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 "components/update_client/request_sender.h"
 
 #include <memory>
 
 #include "base/macros.h"
 #include "base/memory/ptr_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/strings/string_util.h"
 #include "base/test/scoped_task_scheduler.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/update_client/test_configurator.h"
 #include "components/update_client/url_request_post_interceptor.h"
 #include "net/url_request/url_fetcher.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 236 matching lines @@
       << post_interceptor_1_->GetRequestsAsString();
   EXPECT_EQ(1, post_interceptor_1_->GetCount())
       << post_interceptor_1_->GetRequestsAsString();
 
   EXPECT_STREQ("test", post_interceptor_1_->GetRequests()[0].c_str());
   EXPECT_EQ(RequestSender::kErrorResponseNotTrusted, error_);
   EXPECT_TRUE(response_.empty());
 }
 
 }  // namespace update_client