[MD Extensions] Add the source type on the details page for an extension

chrome/browser/resources/md_extensions/detail_view.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('extensions', function() {
   'use strict';
 
   var DetailView = Polymer({
     is: 'extensions-detail-view',
 
@@ skipping 72 matching lines @@
           this.data.id, this.$$('#collect-errors').checked);
     },
 
     /**
      * @param {!chrome.developerPrivate.DependentExtension} item
      * @private
      */
     computeDependentEntry_: function(item) {
       return loadTimeData.getStringF('itemDependentEntry', item.name, item.id);
     },
+
+    /** @private */
+    computeSourceString_: function() {
+      return extensions.getItemSourceString(
+          extensions.getItemSource(this.data));
+    }
   });
 
   return {DetailView: DetailView};
 });