Merge SetPayloadData into constructor. Add TODO to support first_packet_reduction_len Bug: webrtc:9680 Change-Id: I65e771848e0ffe8968cd084840e77afc0152caeb Reviewed-on: https://webrtc-review.googlesource.com/99505 Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Commit-Queue: Danil Chapovalov <danilchap@webrtc.org> Cr-Commit-Position: refs/heads/master@{#24702}