CfC to transition MIX to PR.

Hey folks!

MIX has been sitting around as CR for over two years now. I believe we
agreed in last year's TPAC to advance it once we aligned on behavior for
prefetch (tests
<https://wpt.fyi/results/mixed-content/link-prefetch-tag?label=experimental>),
and to close the `<track>` issue at
https://github.com/w3c/webappsec-mixed-content/issues/13 as WontFix, aiming
for tightening the behavior for `<audio>` and `<video>` in MIX2 rather than
loosening anything in MIX1. I didn't actually follow up on any of that
(except insofar as we did change Chrome's behavior to match Firefox's (and
the spec) for prefetch!).

At this point, I think it's past time that we called MIX done. I've created
https://github.com/w3c/webappsec-mixed-content/issues/15 to discuss
transitioning to PR.

Please comment there within the next two weeks (e.g. by Friday, September
14th) if you have opinions on the matter. :) If the CfC passes, Dan and I
will put together a transition request for the AC to chew on to finalize
the transition.

-mike