Moved from RtpSender to RtpSenderVideo, since currently the
PlayoutDelay extension is used for video only, and configured via
RTPVideoHeader.
Bug: webrtc:7135
Change-Id: Idfcc90cefea83e40a4e79164d7914cdcd50e41fe
Reviewed-on: https://webrtc-review.googlesource.com/c/120357
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26484}