Bug 974160. Robustify reftest. r=cam

This currently seems to fail on b2g aws. I assume because the timeout fires
before the function can run and we end up with two exclamation marks.

--HG--
extra : rebase_source : 7a4d5371c893b4f714df12deca664703a4bc94ac

layout/reftests/svg/text/multiple-x-percentages-3.html

@@ -22,5 +22,4 @@ function doTest() {
 }
 
 window.addEventListener("MozReftestInvalidate", doTest, false);
-setTimeout(doTest, 4000); // fallback for running outside reftest
 </script>