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 <philipp.hancke@googlemail.com>
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563196}
1 file changed