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

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();
   // Promises have microtask timing, so we use setTimeout to explicity force a
   // new task.
   return new Promise(function(resolve, reject) {
     window.setTimeout(resolve, 0);
   });
 }
 
 /**
+ * Replace the current primary element of the test with a new element. Useful
+ * as an alternative to PolymerTest.clearBody() which preserves styling.
+ * @param {Element} element
+ */
+function replaceBody(element) {
+  var body = document.body;
+  var currentBody =
+      body.querySelector('history-app') || body.querySelector('.test-body');
+  body.removeChild(currentBody);
+
+  element.classList.add('test-body');
+  body.appendChild(element);
+}
+
+/**
+ * Replace the document body with a new instance of <history-app>.
+ * @return {HistoryAppElement} The app which was created.
+ */
+function replaceApp() {
+  var app = document.createElement('history-app');
+  app.id = 'history-app';
+  replaceBody(app);
+  return app;
+}
+
+/**
  * Create a fake history result with the given timestamp.
  * @param {number|string} timestamp Timestamp of the entry, as a number in ms or
  * a string which can be parsed by Date.parse().
  * @param {string} urlStr The URL to set on this entry.
  * @return {!HistoryEntry} An object representing a history entry.
  */
 function createHistoryEntry(timestamp, urlStr) {
   if (typeof timestamp === 'string')
     timestamp += ' UTC';
 
@@ skipping 93 matching lines @@
     clientX: xy.x,
     clientY: xy.y,
     buttons: 1,
     shiftKey: true,
   };
 
   element.dispatchEvent(new MouseEvent('mousedown', props));
   element.dispatchEvent(new MouseEvent('mouseup', props));
   element.dispatchEvent(new MouseEvent('click', props));
 }