Add UnifiedPDFPlugin which inherits from the same virtual base class as PDFPlugin.
<rdar://problem/116029997>
Pull request: https://github.com/WebKit/WebKit/pull/18207
Committed 268476@main (b7c083b8e5ef): <https://commits.webkit.org/268476@main> Reviewed commits have been landed. Closing PR #18207 and removing active labels.