[UnifiedPDF] De-hoist "PDFView" coordinate conversion functions back to PDFPlugin
<rdar://problem/118906205>
Pull request: https://github.com/WebKit/WebKit/pull/21030
Committed 271253@main (47696bacc0e0): <https://commits.webkit.org/271253@main> Reviewed commits have been landed. Closing PR #21030 and removing active labels.