Share some adjustTransientZoom() code between TiledCoreAnimationDrawingArea and RemoteLayerTreeDrawingAreaMac
<rdar://problem/103851612>
Pull request: https://github.com/WebKit/WebKit/pull/8168
Committed 258417@main (10939443b246): <https://commits.webkit.org/258417@main> Reviewed commits have been landed. Closing PR #8168 and removing active labels.