I did this instead of just (ab)using the fact that every list item has at least
one counter-increment node because:
* I don't have the bullet frame around by the time we initially compute the
counter increment, which means that I'd need to grow nsBlockFrame / add a
frame property for the list item ordinal, which I think would be unfortunate.
* It feels more consistent with the way regular CSS counters work and with the
way we want ::marker to eventually work.
Differential Revision: https://phabricator.services.mozilla.com/D31990
--HG--
extra : moz-landing-system : lando