Fix chrome://history on Android (check whether element is present first).

$('alertOverlay') is null so the call for classList failed.  Guard the
condition to see if the element exists.

BUG=379194

Review URL: https://codereview.chromium.org/301223009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@273956 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/history/history.js

@@ -996,7 +996,8 @@ HistoryView.prototype.positionNotificationBar = function() {
  * @private
  */
 HistoryView.prototype.clear_ = function() {
-  if ($('alertOverlay').classList.contains('showing'))
+  var alertOverlay = $('alertOverlay');
+  if (alertOverlay && alertOverlay.classList.contains('showing'))
     hideConfirmationOverlay();
 
   this.resultDiv_.textContent = '';