Remove deprecated window.showModalDialog()

ManualTests/js-timers-beneath-modal-dialog.html

-<p>JS timers should be paused while a modal dialog (or alert) is displayed.</p>
-<p>Please disable popup blocker for this test.</p>
+<p>JS timers should be paused while an alert is displayed.</p>
 <div>0</div>
 <script>
 function f()
 {
     var d = document.getElementsByTagName("div")[0];
     d.innerHTML = parseInt(d.innerHTML) + 1;
 }
 
 var timerId = setInterval(f, 100);
 
 alert("Numbers in main window should not increase while this alert is displayed");
-showModalDialog("data:text/html,<p>Numbers in main window should not increase while this modal dialog is displayed</p>");
 document.getElementsByTagName("div")[0].innerHTML = "DONE";
 
 clearInterval(timerId);
 
 </script>