This property can be useful to more subclasses, so let's promote it from Slider to SliderBase.
<rdar://problem/96583766>
Pull request: https://github.com/WebKit/WebKit/pull/2160
Committed 252223@main (780c2fc34af9): <https://commits.webkit.org/252223@main> Reviewed commits have been landed. Closing PR #2160 and removing active labels.