Packet::GetHeader() wasn't copying playout_delay. As result
playout_delay was ignored when flexfec is enabled.
Patch by Rob McCool <rmccool@google.com>
BUG=webrtc:7590
Review-Url: https://codereview.webrtc.org/2899553003
Cr-Commit-Position: refs/heads/master@{#18218}