It's marked as a private header, which is odd for an SPI header. It's imported by one public header (WebDownload.h) and I think it makes sense to just inline it there.
<rdar://problem/128695608>
Pull request: https://github.com/WebKit/WebKit/pull/29078
Committed 279303@main (35683740872f): <https://commits.webkit.org/279303@main> Reviewed commits have been landed. Closing PR #29078 and removing active labels.