Ports demo to native VR API

Hi from Mozilla. This patch makes one of those demos use the VR native APIs in the browser.

You need to run the demo in one of the development builds:

http://vrhelloworld.com/builds/
These should be left as // for display within [https iframes](https://developer.leapmotion.com/gallery/light-pinch-vr).  Makes it harder for dev, I know :(