WebUI cr-dialog: Cancel dialog on any 'popstate' event.

ui/webui/resources/cr_elements/cr_dialog/cr_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 'cr-dialog' is a component for showing a modal dialog. If the
  * dialog is closed via close(), a 'close' event is fired. If the dialog is
  * canceled via cancel(), a 'cancel' event is fired followed by a 'close' event.
  * Additionally clients can inspect the dialog's |returnValue| property inside
  * the 'close' event listener to determine whether it was canceled or just
  * closed, where a truthy value means success, and a falsy value means it was
  * canceled.
  */
 Polymer({
   is: 'cr-dialog',
   extends: 'dialog',
 
+  /** @override */
+  created: function() {
+    // If the active history entry changes (i.e. user clicks back button),
+    // all open dialogs should be cancelled.
+    window.addEventListener('popstate', function() {
+      if (this.open)
+        this.cancel();
+    }.bind(this));
+  },
+
   cancel: function() {
     this.fire('cancel');
     HTMLDialogElement.prototype.close.call(this, '');
   },
 
   /**
    * @param {string=} opt_returnValue
    * @override
    */
   close: function(opt_returnValue) {
     HTMLDialogElement.prototype.close.call(this, 'success');
   },
 
   /** @return {!PaperIconButtonElement} */
   getCloseButton: function() {
     return this.$.close;
   },
 });