This change migrates RTCPSender to use webrtc::Timestamp, preparing
for later improvements regarding bugs.webrtc.org/11581.
Fixed: webrtc:12873
Change-Id: I1159701dc373883367d9b2c86823f8fb59904d55
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/222324
Commit-Queue: Markus Handell <handellm@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34346}