Material PDF: Drastically reduce time required to stamp complex bookmark trees

chrome/browser/resources/pdf/elements/viewer-bookmark/viewer-bookmark.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.
 
 (function() {
   /** Amount that each level of bookmarks is indented by (px). */
   var BOOKMARK_INDENT = 20;
 
   Polymer({
     is: 'viewer-bookmark',
@@ skipping 10 matching lines @@
         observer: 'bookmarkChanged_'
       },
 
       depth: {
         type: Number,
         observer: 'depthChanged'
       },
 
       childDepth: Number,
 
-      childrenShown_: {
+      childrenShown: {
         type: Boolean,
+        reflectToAttribute: true,
         value: false
       }
     },
 
     bookmarkChanged_: function() {
       this.$.expand.style.visibility =
           this.bookmark.children.length > 0 ? 'visible' : 'hidden';
     },
 
     depthChanged: function() {
       this.childDepth = this.depth + 1;
       this.$.item.style.paddingLeft = (this.depth * BOOKMARK_INDENT) + 'px';
     },
 
     onClick: function() {
       if (this.bookmark.hasOwnProperty('page'))
         this.fire('change-page', {page: this.bookmark.page});
     },
 
     toggleChildren: function(e) {
-      this.childrenShown_ = !this.childrenShown_;
-      if (this.childrenShown_)
-        this.$.expand.classList.add('open');
-      else
-        this.$.expand.classList.remove('open');
+      this.childrenShown = !this.childrenShown;
       e.stopPropagation();  // Prevent the above onClick handler from firing.
     }
   });
 })();