[MD Bookmarks] Change selection items from Map to Set.

chrome/browser/resources/md_bookmarks/item.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: 'bookmarks-item',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
@@ skipping 28 matching lines @@
     'click': 'onClick_',
     'dblclick': 'onDblClick_',
   },
 
   /** @override */
   attached: function() {
     this.watch('item_', function(store) {
       return store.nodes[this.itemId];
     }.bind(this));
     this.watch('isSelectedItem_', function(store) {
-      return !!store.selection.items[this.itemId];
+      return !!store.selection.items.has(this.itemId);
     }.bind(this));
 
     this.updateFromStore();
   },
 
   /** @return {BookmarksItemElement} */
   getDropTarget: function() {
     return this;
   },
 
@@ skipping 44 matching lines @@
   },
 
   /**
    * @param {string} url
    * @private
    */
   updateFavicon_: function(url) {
     this.$.icon.style.backgroundImage = cr.icon.getFavicon(url);
   },
 });