This was used for the ondemand mode when we allowed to exit accelerated compositing mode, but now we force always compositing the first time we enter accelerated compositing mode. The only way to exit accelerated compositing is by setting the policy to never, in which case we don't want to keep the previous layer tree host since it's unlikely we enter it again soon.
Pull request: https://github.com/WebKit/WebKit/pull/17853
Committed 269463@main (825a7cc7c424): <https://commits.webkit.org/269463@main> Reviewed commits have been landed. Closing PR #17853 and removing active labels.