Update chrome://device-emulator with newer Polymer guidelines

chrome/browser/resources/chromeos/emulator/device_emulator_pages.js

+// 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.
+
+cr.exportPath('device_emulator', {
+  audioSettings: null,
+  batterySettings: null,
+  bluetoothSettings: null,
+});
+
+Polymer({
+  is: 'device-emulator-pages',
+
+  properties: {
+    selectedPage: {
+      type: Number,
+      value: 0,
+    },
+  },
+
+  ready: function() {
+    for (var page of Polymer.dom(this.$$('neon-animated-pages')).children) {
+      page.initialize();
+      device_emulator[page.id] = page;
+    }
+
+    chrome.send('initializeDeviceEmulator');
+  },
+});