This prepares for future cleanup of how RtpTransportControllerSend is used. Bug: webrtc:10365 Change-Id: Idefc7e60f83819627c83b397949c8434d93491b3 Reviewed-on: https://webrtc-review.googlesource.com/c/124783 Reviewed-by: Stefan Holmer <stefan@webrtc.org> Commit-Queue: Sebastian Jansson <srte@webrtc.org> Cr-Commit-Position: refs/heads/master@{#26923}