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

chrome/browser/resources/md_history/history.js

 // Copyright 2015 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.
 
 // Send the history query immediately. This allows the query to process during
 // the initial page startup.
 chrome.send('queryHistory', ['', 0, 0, 0, RESULTS_PER_PAGE]);
 chrome.send('getForeignSessions');
 
 /** @type {Promise} */
@@ skipping 85 matching lines @@
  */
 function historyDeleted() {
 }
 
 /**
  * Called by the history backend after user's sign in state changes.
  * @param {boolean} isUserSignedIn Whether user is signed in or not now.
  */
 function updateSignInState(isUserSignedIn) {
   waitForAppUpgrade().then(function() {
-    /** @type {HistoryAppElement} */($('history-app'))
-        .updateSignInState(isUserSignedIn);
+    if ($('history-app')) {
+      /** @type {HistoryAppElement} */($('history-app'))
+          .updateSignInState(isUserSignedIn);
+    }
   });
 }