Make listbox-scrollbar-incremental-load.html deterministic

third_party/WebKit/LayoutTests/fast/forms/select/listbox-scrollbar-incremental-load.html

 <p>
     Test for <i><a href="http://bugs.webkit.org/show_bug.cgi?id=13500">http://bugs.webkit.org/show_bug.cgi?id=13500</a>
     REGRESSION: Listbox scrollbar does not reflect actual scrolled position when selected option is added after layout</i>.
 </p>
 <p>
     The list box should be scrolled all the way down, showing the selected option
     &ldquo;Seven&rdquo;. The scroller should be at the bottom of the scroll bar
     to reflect this.
 </p>
 <select size="4">
     <option>One</option>
     <option>Two</option>
     <option>Three</option>
     <option>Four</option>
     <script>
         document.body.offsetTop;
         testRunner.waitUntilDone();
+        // The selection is updated as a separate task, so wait for two frames
+        // to make sure it has run.
         window.requestAnimationFrame(function() {
-          testRunner.notifyDone();
+          window.requestAnimationFrame(function() {
+            testRunner.notifyDone();
+          });
         });
     </script>
     <option>Five</option>
     <option>Six</option>
     <option selected>Seven</option>
 </select>