PushSubscription: Update compatibility for Firefox (#3602)
PushSubscription: Update compatibility for Firefox (#3602)

There were lots of incomplete bits in here for Firefox, as well
as things listed as implemented that are not. These should now
be accurate.

Blame of the WebIDL:
https://hg.mozilla.org/mozilla-central/annotate/0bf136f3ac45ce5e1c5d76c9c89d2fed6d15d0c4/dom/webidl/PushSubscription.webidl#l

Basic implementation:
https://hg.mozilla.org/mozilla-central/annotate/b277f825bc83/dom/webidl/PushSubscription.webidl#l12

Addition of serializer (toJSON()):
https://hg.mozilla.org/mozilla-central/annotate/dda92c46bb23/dom/webidl/PushSubscription.webidl#l39
mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data