Snapshotting of mainframe and subframe PDFs are blank in UI-side compositing mode
<rdar://problem/103160251>
Pull request: https://github.com/WebKit/WebKit/pull/7431
Committed 258399@main (30db210558cf): <https://commits.webkit.org/258399@main> Reviewed commits have been landed. Closing PR #7431 and removing active labels.