[iOS] Flash of black while exiting fullscreen on youtube.com
<rdar://problem/117708957>
Pull request: https://github.com/WebKit/WebKit/pull/24997
Committed 275240@main (c1124e4e8f4c): <https://commits.webkit.org/275240@main> Reviewed commits have been landed. Closing PR #24997 and removing active labels.