unified plan: ice connection state goes to disconnected when adding a new m-line

• Component: WebRTC
• Status: RESOLVED CONFIGURATION CHANGED

[Arne Gleditsch]

Safari TP 77 seems to exhibit the same (incorrect) behavior as Chrome when adding new m-lines to an existing peer connection: https://bugs.chromium.org/p/chromium/issues/detail?id=909684

To Reproduce:
* Go to https://webrtc.github.io/samples/src/content/peerconnection/pc1/
* Make a call (having Develop -> Experimental Features -> WebRTC Unified Plan enabled)
* Paste into the console:
navigator.mediaDevices.getDisplayMedia({video: true}).then(stream => {
    pc1.addTrack(stream.getTracks()[0], stream);
    pc1.createOffer().then(offer => pc1.setLocalDescription(offer));
});

* Observe that console output indicates that ICE state goes to "disconnected"
* Repeat having Unified Plan disabled, observing that the same does not happen in this scenario.

[youenn fablet]

Unified plan is on by default.