Querying user consent status

[NellWaliczek]

[Disclaimer: This issue is one of several being filed to capture discussions that began either on #638, on #689, or at the most recent F2F]

This issue is dependent on the outcome of #720. If it is possible to request user consent in the middle of an immersive session, developers will likely need a way to determine if an API call will trigger a consent prompt so that they can explain to the end user that this will happen. For non-XR features, the Permissions API services this need via the query() API.

This issue tracks the need to determine if WebXR should allow for the ability to check if a prompt will be shown. It also track the discussion about what (if any) relationship WebXR should have with the existing Permissions API.

(Note, this issue will probably be one of the later ones that gets solved out of this batch as it is dependent on a number of others landing first)

[ddorwin]

The answer likely depends on the answer to #725.

[Manishearth]

/agenda to be discussed alongside #725

[Manishearth]

We seem to be leaning towards the direction of relying on the existing permissions API to some extent at least, so there's a good chance this issue will be unnecessary, based on what we settle on.

[probot-label]

This issue is fixed by PR #900