chromeos: Add Power device page to chrome://md-settings.

chrome/test/data/webui/settings/device_page_tests.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.define('device_page_tests', function() {
   /** @enum {string} */
   var TestNames = {
     DevicePage: 'device page',
     Display: 'display',
     Keyboard: 'keyboard',
@@ skipping 570 matching lines @@
       suite('no power settings', function() {
         test('power row hidden', function() {
           assertEquals(null, devicePage.$$('#powerRow'));
           assertEquals(0,
               settings.DevicePageBrowserProxyImpl.getInstance()
               .updatePowerStatusCalled_);
         });
       });
 
       suite('power settings', function() {
-        var powerRow;
+        var powerPage;
+        var powerSourceRow;
         var powerSourceWrapper;
         var powerSourceSelect;
 
         suiteSetup(function() {
           // Always show power settings.
           loadTimeData.overrideValues({
             enablePowerSettings: true,
           });
         });
 
         setup(function() {
-          powerRow = assert(devicePage.$$('#powerRow'));
-          powerSourceWrapper =
-              assert(powerRow.querySelector('.md-select-wrapper'));
-          powerSourceSelect = assert(devicePage.$$('#powerSource'));
-          assertEquals(1,
-              settings.DevicePageBrowserProxyImpl.getInstance()
-              .updatePowerStatusCalled_);
+          return showAndGetDeviceSubpage('power', settings.Route.POWER)
+              .then(function(page) {
+                powerPage = page;
+                powerSourceRow = assert(powerPage.$$('#powerSourceRow'));
+                powerSourceWrapper =
+                    assert(powerSourceRow.querySelector('.md-select-wrapper'));
+                powerSourceSelect = assert(powerPage.$$('#powerSource'));
+                assertEquals(
+                    1,
+                    settings.DevicePageBrowserProxyImpl.getInstance()
+                        .updatePowerStatusCalled_);
+              });
         });
 
         test('power sources', function() {
           var batteryStatus = {
             charging: false,
             calculating: false,
             percent: 50,
             statusText: '5 hours left',
           };
           cr.webUIListenerCallback(
@@ skipping 180 matching lines @@
         assert(waitingDiv.hidden);
         assert(!selectAppDiv.hidden);
       });
     });
   });
 
   return {
     TestNames: TestNames
   };
 });