Update Packet::GetHeader() to copy playout_delay
Update Packet::GetHeader() to copy playout_delay

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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#18218}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc