The point of this change is to make it possible to create the congestion
controller as part of creating RtpTransportController, later pass it to the
constructor of Call, and then let Call register itself as an observer.
BUG=webrtc:7135
Review-Url: https://codereview.webrtc.org/2795643002
Cr-Commit-Position: refs/heads/master@{#17504}