This makes the style system pass flags down to GetMetricsForCSSUnits to specify
whether it needs the 'ch' or 'ic' units, which may trigger downloading additional
font resources.
There should be no change in user-visible behavior, except by observing (e.g. in
the devtools network panel) what resources end up being fetched in an example
like the reporter's.
Differential Revision: https://phabricator.services.mozilla.com/D246780
c68d97e633