[Media Router WebUI] Remove unnecessary default property values.

chrome/browser/resources/media_router/elements/issue_banner/issue_banner.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.
 
 // This Polymer element is used to show information about issues related
 // to casting.
 Polymer({
   is: 'issue-banner',
 
   properties: {
     /**
      * Maps an issue action type to the resource identifier of the text shown
      * in the action button.
      * This is a property of issue-banner because it is used in tests. This
      * property should always be set before |issue| is set or updated.
      * @private {!Array<string>}
      */
     actionTypeToButtonTextResource_: {
       type: Array,
       readOnly: true,
       value: function() {
         return ['dismissButton', 'learnMoreText'];
       },
     },
 
     /**
      * The text shown in the default action button.
-     * @private {string}
+     * @private {string|undefined}
      */
     defaultActionButtonText_: {
       type: String,
-      value: '',
     },
 
     /**
      * The issue to show.
-     * @type {?media_router.Issue}
+     * @type {?media_router.Issue|undefined}
      */
     issue: {
       type: Object,
-      value: null,
       observer: 'updateActionButtonText_',
     },
 
     /**
      * The text shown in the secondary action button.
-     * @private {string}
+     * @private {string|undefined}
      */
     secondaryActionButtonText_: {
       type: String,
-      value: '',
     },
   },
 
   behaviors: [
     I18nBehavior,
   ],
 
   /**
    * @param {?media_router.Issue} issue
    * @return {boolean} Whether or not to hide the blocking issue UI.
@@ skipping 77 matching lines @@
         secondaryText =
             this.i18n(this.actionTypeToButtonTextResource_[
                 this.issue.secondaryActionType]);
       }
     }
 
     this.defaultActionButtonText_ = defaultText;
     this.secondaryActionButtonText_ = secondaryText;
   },
 });