This will allow the RTP-related subclasses of RtcEvent keep an rtp::Packet for the header, rather than hold the heaver, and at the moment unnecessary for logging, RtpPacketReceived/RtpPacketToSend.
BUG=webrtc:8111
Review-Url: https://codereview.webrtc.org/3013023004
Cr-Commit-Position: refs/heads/master@{#19812}