In preparation to get cursor updates working for UnifiedPDFPlugin, we should move `PDFPlugin::notifyCursorChanged` up into the `PDFPluginBase` structure. This is a trivial amount of work that doesn't need specialization between the two different PDF plugin types.
<rdar://problem/118483364>
Pull request: https://github.com/WebKit/WebKit/pull/20576
Committed 270863@main (ce5a85abac39): <https://commits.webkit.org/270863@main> Reviewed commits have been landed. Closing PR #20576 and removing active labels.