Remove ViERTP_RTCP usage in VideoSendStream.

Replaces interface usage with direct calls on ChannelGroup, ViEEncoder
and ViEChannel removing a layer of indirection.

BUG=1695
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43319004

Cr-Commit-Position: refs/heads/master@{#9088}
3 files changed