Make DOMFormData a subclass of ContextDestructionObserver
<rdar://problem/104659126>
Pull request: https://github.com/WebKit/WebKit/pull/9140
Committed 259431@main (4e04f1a26a82): <https://commits.webkit.org/259431@main> Reviewed commits have been landed. Closing PR #9140 and removing active labels.