This adds a start bitrate field to TargetRateConstraints since this is
required in VideoSendStreamTests. This also reduces the differences
between the old SendSideCongestionController and the new TaskQueue
based version.
Bug: webrtc:9586
Change-Id: I5d3d1414e9d30b51723c911a0bf2e96b876c04e5
Reviewed-on: https://webrtc-review.googlesource.com/92624
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24384}