The helpers intended to replace and deprecate BuildRtpHeader when
RtpSenderAudio/RtpSenderVideo will be updated to pass RtpPacket class
instead of raw buffer for sending.
BUG=webrtc:5261
R=sprang@webrtc.org
Review URL: https://codereview.webrtc.org/2303283002 .
Cr-Commit-Position: refs/heads/master@{#14051}