Capability detection for referrer-policy?

[hillbrad]

As-is, I can't see a way to do capability detection for referrer-policy. Many sites use redirectors to filter referrers today from outbound links in order to protect user privacy, etc. These are round-trips that could be eliminated, but only if the site could know with confidence that referrer policy was supported. UA-sniffing is horrible, so it would be nice if we could add a DOM property or other way to test the policy in-force.