We need to assert in the web process that a committed scrolling tree is valid, which will make it easier to track down causes of MESSAGE_CHECKs related to invalid scrolling trees.
<rdar://problem/108769471>
Pull request: https://github.com/WebKit/WebKit/pull/13350
Committed 263620@main (46572d8428cb): <https://commits.webkit.org/263620@main> Reviewed commits have been landed. Closing PR #13350 and removing active labels.