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/media/base/mediachannel.h b/talk/media/base/mediachannel.h
index 92d0fcb..fe223bb 100644
--- a/talk/media/base/mediachannel.h
+++ b/talk/media/base/mediachannel.h
@@ -927,6 +927,10 @@
   std::vector<DataReceiverInfo> receivers;
 };
 
+struct RtcpParameters {
+  bool reduced_size = false;
+};
+
 template <class Codec>
 struct RtpParameters {
   virtual std::string ToString() const {
@@ -941,6 +945,7 @@
   std::vector<Codec> codecs;
   std::vector<RtpHeaderExtension> extensions;
   // TODO(pthatcher): Add streams.
+  RtcpParameters rtcp;
 };
 
 template <class Codec, class Options>