Add missing IWYU message_loop.h includes.

components/update_client/update_checker_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/update_checker.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/files/file_util.h"
 #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/test/scoped_task_scheduler.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/version.h"
 #include "build/build_config.h"
 #include "components/prefs/testing_pref_service.h"
 #include "components/update_client/component.h"
 #include "components/update_client/persisted_data.h"
 #include "components/update_client/test_configurator.h"
@@ skipping 532 matching lines @@
   EXPECT_EQ(1, post_interceptor_->GetHitCount())
       << post_interceptor_->GetRequestsAsString();
   ASSERT_EQ(1, post_interceptor_->GetCount())
       << post_interceptor_->GetRequestsAsString();
 
   EXPECT_EQ(0, error_);
   EXPECT_STREQ("this", component->action_run_.c_str());
 }
 
 }  // namespace update_client