In anticipation of selection tracking support in UnifiedPDFPlugin, let's factor out PDFPlugin::notifySelectionChanged into a common space.
<rdar://problem/122179410>
Pull request: https://github.com/WebKit/WebKit/pull/23755
Committed 274005@main (749aa4442b2f): <https://commits.webkit.org/274005@main> Reviewed commits have been landed. Closing PR #23755 and removing active labels.