"Unified PDF" will control whether we use the new PDF plugin architecture, which we'll use across platforms (hence "Unified").
<rdar://problem/115757129>
Pull request: https://github.com/WebKit/WebKit/pull/17950
Committed 268364@main (8aa2481bf186): <https://commits.webkit.org/268364@main> Reviewed commits have been landed. Closing PR #17950 and removing active labels.