MD History: Improve test helpers, update synced tabs tests to use them

chrome/browser/resources/md_history/app.crisper.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 PromiseResolver is a helper class that allows creating a
  * Promise that will be fulfilled (resolved or rejected) some time later.
  *
  * Example:
  *  var resolver = new PromiseResolver();
@@ skipping 7727 matching lines @@
  * 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) {
@@ skipping 6669 matching lines @@
       lastVisitTime, maxResults
     ]);
   },
 
   /** @return {number} */
   getSelectedItemCount: function() {
     return this.getSelectedList_().selectedPaths.size;
   },
 
   unselectAllItems: function(count) {
-    this.getSelectedList_().unselectAllItems(count);
+    var selectedList = this.getSelectedList_();
+    if (selectedList)
+      selectedList.unselectAllItems(count);
   },
 
   /**
    * Delete all the currently selected history items. Will prompt the user with
    * a dialog to confirm that the deletion should be performed.
    */
   deleteSelectedWithPrompt: function() {
     if (!loadTimeData.getBoolean('allowDeletingHistory'))
       return;
     this.$.dialog.get().then(function(dialog) {
@@ skipping 861 matching lines @@
     return selectedPage;
   },
 
   /** @private */
   closeDrawer_: function() {
     var drawer = this.$$('#drawer');
     if (drawer)
       drawer.close();
   },
 });