Add missing IWYU message_loop.h includes.

chrome/browser/chromeos/input_method/input_method_engine_browsertests.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 <stddef.h>
 
 #include "base/bind_helpers.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 #include "chrome/browser/extensions/extension_browsertest.h"
 #include "content/public/test/browser_test_utils.h"
 #include "content/public/test/test_utils.h"
 #include "extensions/browser/api/virtual_keyboard_private/virtual_keyboard_delegate.h"
 #include "extensions/browser/api/virtual_keyboard_private/virtual_keyboard_private_api.h"
 #include "extensions/browser/process_manager.h"
 #include "extensions/common/manifest_handlers/background_info.h"
 #include "extensions/test/extension_test_message_listener.h"
@@ skipping 1149 matching lines @@
         ui::TEXT_INPUT_FLAG_NONE);
     engine_handler->FocusIn(context);
     ASSERT_TRUE(focus_listener.WaitUntilSatisfied());
     ASSERT_TRUE(focus_listener.was_satisfied());
   }
 }
 
 }  // namespace
 }  // namespace input_method
 }  // namespace chromeos