UnifiedPDF: Implement link following (external links)
<rdar://problem/118550787>
Pull request: https://github.com/WebKit/WebKit/pull/23985
Committed 274205@main (869c37388305): <https://commits.webkit.org/274205@main> Reviewed commits have been landed. Closing PR #23985 and removing active labels.
break the builds, tracking revert in rdar://122473305