webrtc wpt: check signalingState before addIceCandidate

in order to avoid issues with calling addIceCandidate
when the peerconnection is closed

BUG=836871

Change-Id: I698f5365a4e1b72a812fbc5e3adf5e51586718a6
Reviewed-on: https://chromium-review.googlesource.com/1079550
Commit-Queue: Philipp Hancke <[email protected]>
Commit-Queue: Henrik Boström <[email protected]>
Reviewed-by: Henrik Boström <[email protected]>
Cr-Commit-Position: refs/heads/master@{#563196}

webrtc/RTCPeerConnection-helper.js

@@ -229,7 +229,7 @@ function exchangeIceCandidates(pc1, pc2) {
       // There is ongoing discussion on w3c/webrtc-pc#1213
       // that there should be an empty candidate string event
       // for end of candidate for each m= section.
-      if(candidate) {
+      if(candidate && remotePc.signalingState !== 'closed') {
         remotePc.addIceCandidate(candidate);
       }
     });