Update toExistingAtomString() to return an AtomString instead of a RefPtr<AtomStringImpl> as it is a bit less awkward to use.
Pull request: https://github.com/WebKit/WebKit/pull/440
Committed r293627 (250133@main): <https://commits.webkit.org/250133@main> Reviewed commits have been landed. Closing PR #440 and removing active labels.
<rdar://problem/92538851>