Also make it optional and use default value, if optional is not specified. It is done also for next refactoring, that will introduce ability to override network simulation layer. Bug: webrtc:9630 Change-Id: I88cf1f9c70857f3299b5c3e9580a98570768e129 Reviewed-on: https://webrtc-review.googlesource.com/96121 Reviewed-by: Erik Språng <sprang@webrtc.org> Reviewed-by: Patrik Höglund <phoglund@webrtc.org> Commit-Queue: Artem Titov <titovartem@webrtc.org> Cr-Commit-Position: refs/heads/master@{#24454}