With OffscreenCanvas the font set cache can be used from worker threads, so move it to FontCache to ensure there's one cache per thread instead.
Pull request: https://github.com/WebKit/WebKit/pull/3530
Committed 253672@main (115d9297ff1a): <https://commits.webkit.org/253672@main> Reviewed commits have been landed. Closing PR #3530 and removing active labels.
<rdar://problem/99019330>