Tune font prewarming on improve page load performance.
Pull request: https://github.com/WebKit/WebKit/pull/15652
<rdar://problem/112291907>
Committed 266188@main (fab410336356): <https://commits.webkit.org/266188@main> Reviewed commits have been landed. Closing PR #15652 and removing active labels.
Re-opened since this is blocked by bug 261433