Add missing IWYU message_loop.h includes.

chrome/browser/app_controller_mac_browsertest.mm

 // 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.
 
 #import <Carbon/Carbon.h>
 #import <Cocoa/Cocoa.h>
 #import <Foundation/Foundation.h>
 #import <Foundation/NSAppleEventDescriptor.h>
 #import <objc/message.h>
 #import <objc/runtime.h>
 #include <stddef.h>
 
 #include "base/command_line.h"
 #include "base/mac/foundation_util.h"
 #include "base/mac/scoped_nsobject.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/threading/thread_restrictions.h"
 #include "chrome/app/chrome_command_ids.h"
 #import "chrome/browser/app_controller_mac.h"
 #include "chrome/browser/apps/app_browsertest_util.h"
 #include "chrome/browser/bookmarks/bookmark_model_factory.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/history/history_service_factory.h"
@@ skipping 685 matching lines @@
   ui_test_utils::NavigateToURL(browser3, test_url1);
   EXPECT_EQ(g_handoff_url, GURL());
 
   // Activate the original browser window.
   Browser* browser1 = active_browser_list->get(0);
   browser1->window()->Show();
   EXPECT_EQ(g_handoff_url, test_url2);
 }
 
 }  // namespace