Add FlexfecReceiveStream.

webrtc/config.cc

Index: webrtc/config.cc
diff --git a/webrtc/config.cc b/webrtc/config.cc
index 0331c0a8332977af9ed035cb73ff1c0cef4a51ca..a797fed02c6d44ae26e17a01e71a9cf2fece3b6b 100644
--- a/webrtc/config.cc
+++ b/webrtc/config.cc
@@ -31,6 +31,15 @@ std::string UlpfecConfig::ToString() const {
   return ss.str();
 }
 
+std::string FlexfecConfig::ToString() const {
+  std::stringstream ss;
+  ss << "{flexfec_payload_type: " << flexfec_payload_type;
+  ss << ", flexfec_ssrc: " << flexfec_ssrc;
+  ss << ", protected_media_ssrc: " << protected_media_ssrc;
+  ss << '}';
+  return ss.str();
+}
+
 std::string RtpExtension::ToString() const {
   std::stringstream ss;
   ss << "{uri: " << uri;