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}