Bug 1415787 - Wait for focus before proceeding test. r=daoshengmu

To process focus handling deterministic, we need to wait for focus before
proceeding test.

MozReview-Commit-ID: 78EWlA3QuWM

--HG--
extra : rebase_source : 6473b0e1d364f9d3f8a5c43f82ee25be54a478bf

dom/svg/test/test_tabindex.html

@@ -96,7 +96,8 @@ function main()
   SimpleTest.finish();
 }
 
-window.addEventListener("load", main);
+SimpleTest.waitForFocus(main);
+
 </script>
 </pre>
 </body>