Add missing IWYU message_loop.h includes.

content/shell/browser/layout_test/blink_test_controller.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 "content/shell/browser/layout_test/blink_test_controller.h"
 
 #include <stddef.h>
 
 #include <iostream>
 #include <set>
 #include <utility>
 
 #include "base/base64.h"
 #include "base/callback.h"
 #include "base/command_line.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/stringprintf.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "content/public/browser/devtools_agent_host.h"
 #include "content/public/browser/dom_storage_context.h"
@@ skipping 978 matching lines @@
   }
   DCHECK(layout_test_control_map_[frame].get());
   return layout_test_control_map_[frame].get();
 }
 
 void BlinkTestController::HandleLayoutTestControlError(RenderFrameHost* frame) {
   layout_test_control_map_.erase(frame);
 }
 
 }  // namespace content