Add `negotiated` to RTCDataChannel in Firefox 68 (#4324)
Add `negotiated` to RTCDataChannel in Firefox 68 (#4324)

FX 68 added the negotiated property to RTCDataChannel.

References:
* https://bugzilla.mozilla.org/show_bug.cgi?id=1529695
chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data