1. Rename FecController to FecControllerPlrBased.
2. Introduce FecControllerRplrBased - a version of FecController that makes its decision based on RPLR instead of PLR.
BUG=webrtc:7058
Review-Url: https://codereview.webrtc.org/2672933003
Cr-Commit-Position: refs/heads/master@{#17373}