[MD History] Factor out a common HistoryListBehavior.

chrome/test/data/webui/md_history/test_util.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.
 
 /*
  * Waits for queued up tasks to finish before proceeding. Inspired by:
  * https://github.com/Polymer/web-component-tester/blob/master/browser/environment/helpers.js#L97
  */
 function flush() {
   Polymer.dom.flush();
@@ skipping 57 matching lines @@
  */
 function createHistoryInfo(searchTerm) {
   return {
     finished: true,
     hasSyncedResults: false,
     queryEndTime: 'Monday',
     queryStartTime: 'Tuesday',
     term: searchTerm || ''
   };
 }
+
+/**
+ * @param {Element} element
+ * @param {string} selector
+ * @return {Element}
+ */
+function polymerSelectAll(element, selector) {
+  return Polymer.dom(element.root).querySelectorAll(selector);
+}
+
+/**
+ * Returns a promise which is resolved when |eventName| is fired on |element|
+ * and |predicate| is true.
+ * @param {HTMLElement} element
+ * @param {string} eventName
+ * @param {function(Event): boolean} predicate
+ * @return {Promise}
+ */
+function waitForEvent(element, eventName, predicate) {
+  if (!predicate)
+    predicate = function() { return true; };
+
+  return new Promise(function(resolve) {
+    var listener = function(e) {
+      if (!predicate(e))
+        return;
+
+      resolve();
+      element.removeEventListener(eventName, listener);
+    }
+
+    element.addEventListener(eventName, listener);
+  });
+}