Enabling rtcp-rsize negotiation and fixing some issues with it.

Sending of reduced size RTCP packets should be enabled only if it's
enabled in the send parameters (which corresponds to the remote description).

Since the RTCPReceiver's RtcpMode isn't used at all, I removed it to ease
confusion.

BUG=webrtc:4868
R=pbos@webrtc.org, pthatcher@google.com, pthatcher@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1713493003 .

Cr-Original-Commit-Position: refs/heads/master@{#12057}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5f0b83b7fb6de95f33f20656e969e0280d72c315
9 files changed