Bug 1890204 - Ensure font entry's unitsPerEm and font extents are initialized...

Bug 1890204 - Ensure font entry's unitsPerEm and font extents are initialized when gfxFont is created. r=gfx-reviewers,lsalzman

This means that by the time we potentially call GetFontExtents() when drawing,
the extents fields are guaranteed to have been been initialized, and there's no
risk of the (read-only) access here racing with setting them in UnitsPerEm().

Differential Revision: https://phabricator.services.mozilla.com/D206920