They can't be forward declared, but they don't need to be if they are only serialized by their containing class.
<rdar://problem/100788834>
Pull request: https://github.com/WebKit/WebKit/pull/5009
Committed 255154@main (58c21888e821): <https://commits.webkit.org/255154@main> Reviewed commits have been landed. Closing PR #5009 and removing active labels.