Add missing IWYU message_loop.h includes.

content/child/child_thread_impl.cc

 // Copyright (c) 2012 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/child/child_thread_impl.h"
 
 #include <signal.h>
 #include <string>
 #include <utility>
 
 #include "base/base_switches.h"
 #include "base/command_line.h"
 #include "base/debug/alias.h"
 #include "base/debug/leak_annotations.h"
 #include "base/debug/profiler.h"
 #include "base/lazy_instance.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/message_loop/timer_slack.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/process/process.h"
 #include "base/process/process_handle.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/synchronization/condition_variable.h"
 #include "base/synchronization/lock.h"
@@ skipping 800 matching lines @@
   Listener* route = router_.GetRoute(routing_id);
   if (route)
     route->OnAssociatedInterfaceRequest(name, request.PassHandle());
 }
 
 bool ChildThreadImpl::IsInBrowserProcess() const {
   return static_cast<bool>(browser_process_io_runner_);
 }
 
 }  // namespace content