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}