MD Bookmarks: Enable the delete button in the toolbar overlay

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar_selection_overlay.js

 // Copyright 2017 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.
 
 /**
  * @fileoverview Element which displays the number of selected items with
  * Cancel/Delete buttons, designed to be used as an overlay on top of
  * <cr-toolbar>. See <history-toolbar> for an example usage.
  */
 
 Polymer({
   is: 'cr-toolbar-selection-overlay',
 
   properties: {
     deleteLabel: String,
 
     cancelLabel: String,
 
     selectionLabel: String,
   },
 
+  /** @return {PaperButtonElement} */
+  get deleteButton() {
+    return this.$.delete;
+  },
+
   /** @private */
   onClearSelectionTap_: function() {
     this.fire('clear-selected-items');
   },
 
   /** @private */
   onDeleteTap_: function() {
     this.fire('delete-selected-items');
   },
 });