Wire up BitrateAllocation to be sent as RTCP TargetBitrate

This is the video parts of https://codereview.webrtc.org/2531383002/
Wire up BitrateAllocation to be sent as RTCP TargetBitrate

BUG=webrtc:6301

Review-Url: https://codereview.webrtc.org/2541303003
Cr-Original-Commit-Position: refs/heads/master@{#15359}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1a646ee522d614dc19b3022b8857cc1a37ec47b4
12 files changed