Fix RTCPeerConnection

[teoli2003]

Added two missing events: onicegatheringstatechange_event and icecandidateerror_event(I took their compat info form onicegatheringstatechange and onicecadidateerror that were both existing.
Added mdn and spec urls for RTCPeerConnection.getTranceivers.

[foolip]

I took their compat info form onicegatheringstatechange and onicecadidateerror that were both existing

@teoli2003 this is often correct, but depressingly often it's not. See #7545 for more complexities than you're probably interested in regarding events.

My advice when adding onfoo and foo_event entries would be:

• First work out when the event can be fired and write a test for that, then use BrowserStack to find out when it was first supported
• Then check if the onfoo property was added in the same version
• If it wasn't, add notes about how only one of the methods of listening for the event work for some range of versions, and cry a little for web developers

Digging through implementation source is often faster than the testing approach, but has many pitfalls of its own. In particular, it's hard to be confident whether some code path firing an event is behind a flag or not.

[foolip]

The Firefox + Firefox for Android data in https://developer.mozilla.org/en-US/docs/Web/API/RTCPeerConnection#browser_compatibility looks pretty implausible, suggesting that the unprefixed RTCPeerConnection was available on desktop since 22, but that on Android it was prefixed as mozRTCPeerConnection from 22 to 43, and unprefixed in 44.

That's probably wrong, and the likely error is being spread around more here.

https://mdn-bcd-collector.appspot.com/tests/api/mozRTCPeerConnection can be used to figure out when the prefixed support was added and removed, if you have the appetite for it.

[teoli2003]

The unprefixing happened in Firefox 44 in the code: https://bugzilla.mozilla.org/show_bug.cgi?id=1155923. So RTCPeerConnection on Fx 22 is impossible.

Note that the removal of the unprefixed version has not happened yet: https://bugzilla.mozilla.org/show_bug.cgi?id=1531812.

I'm adding a commit reflecting this.

[foolip]

I've also sent foolip/mdn-bcd-collector#1246 to improve the collector's ability to detect early support of RTCPeerConnection members.

[foolip]

@teoli2003 would you like to give this a final push? I'm happy to review again :)

[teoli2003]

(Thanks for reminding me of this PR, lost in the list of those waiting review)

[foolip]

There's still lots of Firefox Android data claiming 44 where the Firefox data says something else. This adds one such error, but I'll send a follow-up to fix them all.

[teoli2003]

Thanks!