fix `window.performance.now` polyfill (bug 1090572)

bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1090572

test case:
http://jsbin.com/vihez/1/

test in IE 9:
http://netrenderer.com/index.php

This isn't as good as the native `performance.now()` but I suppose we know that thus the polyfill.