`window.internals.scrollingTreeAsText()` doesn't work on macOS with UI-side compositing; we can use `uiController.scrollingTreeAsText` instead, but need to know when to use it.
<rdar://problem/104681297>
Pull request: https://github.com/WebKit/WebKit/pull/9171
Committed 259457@main (e4fa5c5f61bb): <https://commits.webkit.org/259457@main> Reviewed commits have been landed. Closing PR #9171 and removing active labels.