Doing this at compile time would save a bit of our startup time, which
I've promised to do since @counter-style was initially implemented, see
bug 966166 comment 103 (the paragraph with "constexpr").
Also, having this implemented here makes using atom instead of string
on name of builtin counter styles easier, for later patches.
MozReview-Commit-ID: C9HYcuShBQv
--HG--
extra : rebase_source : 273eb3d23ee770722d27b26aafaa291d7375b8ee
Xidorn Quan