onpushsubscriptionchange is not supported in Chrome and Firefox

[saschanaz]

Same as PushSubscriptionChangeEvent, at least per https://mdn-bcd-collector.appspot.com/tests/.

[queengooborg]

The data for the event interface itself was fixed in #10763, #10863, and #11322! The event handler does seem to be supported in Firefox as reported by the collector, however. We'll probably need to do some more digging into this one.

[Elchi3]

https://bugzilla.mozilla.org/show_bug.cgi?id=1157108 and https://chromestatus.com/feature/6242325854420992 also say onpushsubscriptionchange/ the pushsubscriptionchange event is supported in Firefox. I guess we can close this given the interface was fixed.