Clean up code related to how TiledCoreAnimationDrawingArea gets layers for zooming and shadow
<rdar://problem/101575253>
Pull request: https://github.com/WebKit/WebKit/pull/5799
Committed 256026@main (5c7651583d44): <https://commits.webkit.org/256026@main> Reviewed commits have been landed. Closing PR #5799 and removing active labels.