The original implementation of the unprefixed API tried to make as little change as possible to the prefixed API. The only difference between the two methods is the `isFullyActive()` check. Since we successfully unified that check for `Element::requestFullscreen` in https://commits.webkit.org/255551@main, we should also do it for `Document::fullscreenEnabled`.
<rdar://problem/102937855>
Pull request: https://github.com/WebKit/WebKit/pull/7117