A recent refactoring (r13192) introduced a bug where the min transmit
config wasn't being respected. Specifically, if a VideoSendStream was
created without it and the reconfigured, the min transmit bitrate would
not take effect. Probably the other way around as well.
BUG=webrtc::5687
Review-Url: https://codereview.webrtc.org/2106183002
Cr-Commit-Position: refs/heads/master@{#13390}