Local media SSRC is mandatory, but let's give it a default value to
make tests less brittle.
Bug: chromium:1015256
Change-Id: If7f6505482d90651bc58d9b358290c4d43487f4e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/157421
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29534}