Add missing IWYU message_loop.h includes.

chrome/browser/local_discovery/service_discovery_client_mac.mm

 // 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 "chrome/browser/local_discovery/service_discovery_client_mac.h"
 
 #import <arpa/inet.h>
 #import <Foundation/Foundation.h>
 #import <net/if_dl.h>
 #include <stddef.h>
 #include <stdint.h>
 
 #include "base/debug/dump_without_crashing.h"
 #include "base/memory/singleton.h"
+#include "base/message_loop/message_loop.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "net/base/ip_address.h"
 #include "net/base/ip_endpoint.h"
 
 using local_discovery::ServiceWatcherImplMac;
 using local_discovery::ServiceResolverImplMac;
@@ skipping 476 matching lines @@
   container_->OnResolveUpdate(local_discovery::ServiceResolver::STATUS_SUCCESS);
 }
 
 - (void)netService:(NSNetService*)sender
         didNotResolve:(NSDictionary*)errorDict {
   container_->OnResolveUpdate(
       local_discovery::ServiceResolver::STATUS_REQUEST_TIMEOUT);
 }
 
 @end