Use RtcpPacket to send REMB in RtcpSender
Use RtcpPacket to send REMB in RtcpSender

BUG=webrtc:2450
R=asapersson@webrtc.org

Review URL: https://codereview.webrtc.org/1290573004 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9722}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 35ab4baa20a730de71b390008900a16563cbbe8e

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