REFERRER: Clean up the 'determine' algorithm.

Hixie noted that we do a poor job specifying where we get the URL we're
eventually going to use to generate the referrer[1]. This patch attempts
to explain the logic in less hand-wavey ways that don't just pull URLs
out of thin air.

[1]: http://lists.w3.org/Archives/Public/public-webappsec/2014Jul/0101.html

Author: mikewest