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}