Before this change, the configuration logic in FlexfecReceiveStream
tried to make unsupported configurations work, e.g., by dropping the
protection of some media streams when multiple media streams were
protected by a single FlexFEC stream. This CL makes the configuration logic
return more errors on such unsupported configurations.
This harmonizes the logic with the new configuration logic in
VideoSendStream, for the FlexfecSender.
BUG=webrtc:5654
Review-Url: https://codereview.webrtc.org/2499963002
Cr-Commit-Position: refs/heads/master@{#15083}
43c31e7afe