Implement the code to resolve the track and thumb color from the scrollbar-color property. This will have no behaviour changes but it neccessary plumbing for the actual implementation.
Pull request: https://github.com/WebKit/WebKit/pull/16664
<rdar://problem/114189945>
Committed 267260@main (4045ef1224de): <https://commits.webkit.org/267260@main> Reviewed commits have been landed. Closing PR #16664 and removing active labels.