This changes -moz-bool-pref from @supports to @media, bringing various
improvements:
* @media (-moz-bool-pref) properly reacts to dynamic changes.
* We no longer need to parse chrome:// stylesheets in the main thread
(-moz-bool-pref was the only thing that forced us to do that).
This makes privileged and non-privileged stylesheets more similar.
Differential Revision: https://phabricator.services.mozilla.com/D191196
