The UnifedPDF plugin will build a hierarchy of GraphicsLayers; we'll need to be able to host those across the plugin boundary.
<rdar://problem/116601287>
Pull request: https://github.com/WebKit/WebKit/pull/18793
Committed 269029@main (d02b4b36c7de): <https://commits.webkit.org/269029@main> Reviewed commits have been landed. Closing PR #18793 and removing active labels.