as described in JSEP
https://tools.ietf.org/html/rfc8829#section-3.5.2.1
If the MID field is present in a received IceCandidate, it
MUST be used for identification; otherwise, the "m=" section
index is used instead.
BUG=webrtc:12479
Change-Id: I688a5e59024fe8cc6a170c216c6f14536084cfb9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/208100
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#33357}