[UnifiedPDF] Use the scrolling tree for UnifiedPDFPlugin
<rdar://problem/118154855>
Pull request: https://github.com/WebKit/WebKit/pull/20651
Committed 270917@main (34b37bd8fc56): <https://commits.webkit.org/270917@main> Reviewed commits have been landed. Closing PR #20651 and removing active labels.