Permissions API changes to webxr

Hey all,

Just a quick heads up: we're hoping to discuss the proposed permissions API
improvements to webxr <https://github.com/immersive-web/webxr/pull/900/>
tomorrow at the meeting. They're a bit complicated so it's probably worth
it to review them before the meeting, if possible.

The gist of it is that in order to clean up how we do permissions, we're
hoping to use the permissions API underneath our existing XR feature API,
which will let users continue to interact with XR features the way they can
with the existing API, but also expose additional Permissions API
conveniences like Permissions.query().

Thanks,
-Manish