Migrate <cr-dialog> from PaperDialogBehavior to native <dialog>.

chrome/browser/resources/settings/on_startup_page/startup_url_dialog.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.
 
 /**
  * @fileoverview 'settings-startup-url-dialog' is a component for adding
  * or editing a startup URL entry.
  */
 Polymer({
   is: 'settings-startup-url-dialog',
@@ skipping 27 matching lines @@
       this.dialogTitle_ = loadTimeData.getString('onStartupEditPage');
       this.actionButtonText_ = loadTimeData.getString('onStartupEdit');
       this.$.actionButton.disabled = false;
       // Pre-populate the input field.
       this.url_ = this.model.url;
     } else {
       this.dialogTitle_ = loadTimeData.getString('onStartupAddNewPage');
       this.actionButtonText_ = loadTimeData.getString('add');
       this.$.actionButton.disabled = true;
     }
-    this.$.dialog.open();
+    this.$.dialog.showModal();
   },
 
   /** @private */
   onCancelTap_: function() {
     this.$.dialog.close();
   },
 
   /** @private */
   onActionButtonTap_: function() {
     var whenDone = this.model ?
         this.browserProxy_.editStartupPage(this.model.modelIndex, this.url_) :
         this.browserProxy_.addStartupPage(this.url_);
 
     whenDone.then(function(success) {
       if (success)
         this.$.dialog.close();
       // If the URL was invalid, there is nothing to do, just leave the dialog
       // open and let the user fix the URL or cancel.
     }.bind(this));
   },
 
   /** @private */
   validate_: function() {
     this.browserProxy_.validateStartupPage(this.url_).then(function(isValid) {
       this.$.actionButton.disabled = !isValid;
     }.bind(this));
   },
 });