Make listbox-scrollbar-incremental-load.html deterministic

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

Index: third_party/WebKit/LayoutTests/fast/forms/select/listbox-scrollbar-incremental-load.html
diff --git a/third_party/WebKit/LayoutTests/fast/forms/select/listbox-scrollbar-incremental-load.html b/third_party/WebKit/LayoutTests/fast/forms/select/listbox-scrollbar-incremental-load.html
index 73b78c967a3ddf511b757bb752c0baf7bbff46f4..43db5f9d32bc3c62c9016e3a4e555ab28e9cd584 100644
--- a/third_party/WebKit/LayoutTests/fast/forms/select/listbox-scrollbar-incremental-load.html
+++ b/third_party/WebKit/LayoutTests/fast/forms/select/listbox-scrollbar-incremental-load.html
@@ -15,8 +15,12 @@
     <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>