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}