Limiting the `referer` header's length.

Hey folks!

I mentioned https://github.com/whatwg/fetch/issues/903 on the last call,
and I'd like to mention it again. Based on early Chrome telemetry, it looks
like only ~0.01% of requests have a `referer` header longer than ~4k. In
order to whack a specific xsleaks mole
<https://github.com/xsleaks/xsleaks/wiki/Browser-Side-Channels#cache-and-error-events>,
I'd like to strip the header's value down to its origin if it exceeds that
length. My intuition is that this is a safe way to process the header,
which ensures that important parameters, entities, etc. won't be cut off in
the middle.

I've put together a quick PR against Referrer Policy at
https://github.com/w3c/webappsec-referrer-policy/pull/122. I'd appreciate
feedback either there on on the issue noted above. :)

-mike