Add 'mutable' member to Notification JSON
Pull request: https://github.com/WebKit/WebKit/pull/17842
Committed 268066@main (010581c0ec10): <https://commits.webkit.org/268066@main> Reviewed commits have been landed. Closing PR #17842 and removing active labels.
<rdar://problem/115627147>
Follow-up build fix in https://github.com/WebKit/WebKit/pull/17862