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}