Support unbounded reference spaces in WebXR

[Manishearth]

https://immersive-web.github.io/webxr/#dom-xrreferencespacetype-unbounded

I'm not sure if we support any device where this needs to be handled differently from local, if supported at all. It's possible that for now we can get away with doing the same things as local here.

Most devices likely don't support this.

See also: #24194