Add missing IWYU message_loop.h includes.

ui/views/widget/native_widget_mac_unittest.mm

 // Copyright 2014 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 "ui/views/widget/native_widget_mac.h"
 
 #import <Cocoa/Cocoa.h>
 
 #import "base/mac/foundation_util.h"
 #include "base/mac/mac_util.h"
 #import "base/mac/scoped_nsautorelease_pool.h"
 #import "base/mac/scoped_nsobject.h"
 #import "base/mac/scoped_objc_class_swizzler.h"
 #include "base/macros.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/test/test_timeouts.h"
 #include "base/threading/thread_task_runner_handle.h"
 #import "testing/gtest_mac.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #import "ui/base/cocoa/constrained_window/constrained_window_animation.h"
 #import "ui/base/cocoa/window_size_constants.h"
@@ skipping 1906 matching lines @@
 
 - (void)dealloc {
   if (deallocFlag_) {
     DCHECK(!*deallocFlag_);
     *deallocFlag_ = true;
   }
   [super dealloc];
 }
 
 @end