Add missing IWYU message_loop.h includes.

ui/views/mus/screen_mus_unittest.cc

 // Copyright 2016 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 "ui/views/mus/screen_mus.h"
 
 #include "base/command_line.h"
+#include "base/message_loop/message_loop.h"
 #include "ui/display/display_switches.h"
 #include "ui/display/screen.h"
 #include "ui/views/test/scoped_views_test_helper.h"
 #include "ui/views/test/views_test_base.h"
 
 namespace views {
 namespace {
 
 TEST(ScreenMusTest, ConsistentDisplayInHighDPI) {
   base::MessageLoop message_loop(base::MessageLoop::TYPE_UI);
   base::CommandLine::ForCurrentProcess()->AppendSwitchASCII(
       switches::kForceDeviceScaleFactor, "2");
   ScopedViewsTestHelper test_helper;
   display::Screen* screen = display::Screen::GetScreen();
   std::vector<display::Display> displays = screen->GetAllDisplays();
   ASSERT_FALSE(displays.empty());
   for (const display::Display& display : displays) {
     EXPECT_EQ(2.f, display.device_scale_factor());
     EXPECT_EQ(display.work_area(), display.bounds());
   }
 }
 
 }  // namespace
 }  // namespace views