Set padding bit if the TransportFeedback packet contains zero padding.
Also write number of padding elements at the last position of the packet.
Bug: webrtc:10263
Change-Id: I8d17bc0e889f658ac383dec64ddcb95d438c9702
Reviewed-on: https://webrtc-review.googlesource.com/c/122240
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Johannes Kron <kron@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26646}