Adding reduced size RTCP configuration down to the video stream level.

Still waiting to turn on negotiation (in mediasession.cc)
until we verify it's working as expected.

BUG=webrtc:4868

Review URL: https://codereview.webrtc.org/1418123003

Cr-Commit-Position: refs/heads/master@{#10958}
diff --git a/talk/session/media/channel.cc b/talk/session/media/channel.cc
index f4c0e81..588a036 100644
--- a/talk/session/media/channel.cc
+++ b/talk/session/media/channel.cc
@@ -152,6 +152,7 @@
   if (desc->rtp_header_extensions_set()) {
     params->extensions = desc->rtp_header_extensions();
   }
+  params->rtcp.reduced_size = desc->rtcp_reduced_size();
 }
 
 template <class Codec, class Options>