MD Settings: add tests for <controlled-button>

chrome/browser/resources/settings/controls/controlled_button.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.
 
 Polymer({
   is: 'controlled-button',
 
   behaviors: [CrPolicyPrefBehavior, PrefControlBehavior],
 
   properties: {
     /** @private */
     controlled_: {
       type: Boolean,
-      computed: 'computeControlled_(pref)',
+      computed: 'computeControlled_(pref.*)',
       reflectToAttribute: true,
     },
   },
 
   /**
-   * @param {!chrome.settingsPrivate.PrefObject} pref
    * @return {boolean} Whether the button is disabled.
    * @private
    */
-  computeControlled_: function(pref) {
-    return this.isPrefPolicyControlled(pref);
+  computeControlled_: function() {
+    return this.isPrefPolicyControlled(assert(this.pref));
   },
 
   /**
    * @param {!Event} e
    * @private
    */
   onIndicatorTap_: function(e) {
     // Disallow <controlled-button on-tap="..."> when controlled.
     e.preventDefault();
     e.stopPropagation();
   },
 });