Add PDFPluginBase.mm and fix the United Sources fallout
<rdar://problem/115756126>
Pull request: https://github.com/WebKit/WebKit/pull/17948
Pull request: https://github.com/WebKit/WebKit/pull/17952
Committed 268210@main (8c72c9136a20): <https://commits.webkit.org/268210@main> Reviewed commits have been landed. Closing PR #17952 and removing active labels.