This was accidentally added in 253321@main, but not defined anywhere.
<rdar://problem/102579205>
Pull request: https://github.com/WebKit/WebKit/pull/6698
Committed 256912@main (b3f9f9ceef3f): <https://commits.webkit.org/256912@main> Reviewed commits have been landed. Closing PR #6698 and removing active labels.