A TiledBackingClient can currently only handle a single TiledBacking, because TileGridInder is either 0 or 1. The PDF plugin needs a TiledBackingClient to be a client of multiple TiledBackings.
<rdar://problem/128073464>
Committed 278791@main (a9ca2712f944): <https://commits.webkit.org/278791@main> Reviewed commits have been landed. Closing PR #28552 and removing active labels.