Fix java.lang.ClassCastException in requestAnimationFrame

The parameter to a FrameRequestCallback is not an Int, but a `DOMHighResTimeStamp`

    A DOMHighResTimeStamp SHOULD represent a number of milliseconds accurate to a thousandth of a millisecond.

So (at least in Chrome), a java.lang.ClassCastException is thrown immediately upon entry.  This should fix that.