And use it instead of !RunLoop::isMain(), since we really want to check not only that current thread is not the main onem, but that it's the threaded compositor one.
Pull request: https://github.com/WebKit/WebKit/pull/5078
Committed 255258@main (263fc5c51836): <https://commits.webkit.org/255258@main> Reviewed commits have been landed. Closing PR #5078 and removing active labels.