Minor render loop cleanup.

If targeting a specific fps; there's no need for us to call `getTimestamp`
ourselves because the requestAnimationFrame callback passes us in the
frame time (specifically a DOMHighResTimeStamp)

I also made a tweak to the requestAnimationFrame polyfill to use
`getTimestamp` instead of `Date.now` so that we use higher resolution
timers if they are available.
Merge pull request #2298 from AnalyticalGraphicsInc/requestAnimationFrame-cleanup

Minor render loop cleanup.