Don't allocate any RTPSender object for a receive only RtpRtcp module.

This is one step towards separation of send-side and receive-side
processing.

BUG=webrtc:7135

Review-Url: https://codereview.webrtc.org/2740163002
Cr-Original-Commit-Position: refs/heads/master@{#17306}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 14adba77ecdf6b8ec604363c3717def1e60d7b73
7 files changed