The containsFullscreenElement() flag is going to be removed with the top layer refactoring, let's replace some usages beforehand.
<rdar://problem/102870011>
Pull request: https://github.com/WebKit/WebKit/pull/7039
Committed 257258@main (c2ff6770b421): <https://commits.webkit.org/257258@main> Reviewed commits have been landed. Closing PR #7039 and removing active labels.