Set display mode for extensions::AppWindow

chrome/test/data/extensions/fullscreen_app/check_media_feature.js

+// Copyright 2015 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.
+var fullscreen = matchMedia( '(display-mode: fullscreen)' );
+var p = document.createElement("p");
+document.body.appendChild(p);
+if (fullscreen.matches) {
+  p.innerHTML = "(display-mode: fullscreen) matches.";
+  p.style.color = "green";
+} else {
+  p.innerHTML = "(display-mode: fullscreen) does not match.";
+  p.style.color = "red";
+}