CfC to move Mixed Content Level 2 to FPWD

Hi folks,

We talked a bit during TPAC
<https://github.com/w3c/webappsec/blob/master/meetings/2019/2019-09-TPAC-minutes.md#mix2>
about moving MIX2
<https://w3c.github.io/webappsec-mixed-content/level2.html> forward and
there seemed to be general consensus about moving in that direction. I
created https://github.com/w3c/webappsec-mixed-content/issues/25 to discuss
moving this to a first public working draft, please leave any comments you
have about this there.

Relevant to the MIX2 spec, Chrome has been experimenting with different
types of autoupgrades so far, with generally positive results, and is
planning to launch autoupgrades for audio and video resources in version 80
(due to launch in February 2020).

-Carlos