When FEC generation is moved to egress, we'll need to poll bitrates from
there instead of the RtpVideoSender. In preparation, refactoring some
getter methods.
For context, see https://webrtc-review.googlesource.com/c/src/+/173708
Bug: webrtc:11340
Change-Id: Ibc27362361ee9640d9fce676fc8e1093a579344f
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174202
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31214}
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