Bug 1620359 - Don't clear the "uses viewport units" bit when a font that doesn't cause a style change loads. r=jfkthame

This is probably an old-ish bug made more frequent by the font loading
optimizations.

PostRebuildAllStyleData is a bit of a misnomer, but was always calling
ClearCachedData() on the style set, even if we weren't guaranteed to restyle
every element.

This means both wasted work and correctness issues (as the "uses <rare-feature>"
bits are cleared during this call, on the assumption that we'll then visit all
elements and that'd recompute it properly).

For now, unify a bit the different code paths and only clear these bits if we're
guaranteed to restyle all elements.

I should rename this to something better in a follow-up, and ideally also
decouple the ClearCachedData() calls a bit...

Differential Revision: https://phabricator.services.mozilla.com/D65740

--HG--
extra : moz-landing-system : lando