[Cocoa] Migrate OpticalSizingType from an enum to a variant
<rdar://problem/107538476>
Pull request: https://github.com/WebKit/WebKit/pull/12300
Committed 262554@main (f59b6cd00f1c): <https://commits.webkit.org/262554@main> Reviewed commits have been landed. Closing PR #12300 and removing active labels.