Make |rtcp_send_transport| mandatory in FlexfecReceiveStream::Config.

webrtc/call/flexfec_receive_stream.h

Index: webrtc/call/flexfec_receive_stream.h
diff --git a/webrtc/call/flexfec_receive_stream.h b/webrtc/call/flexfec_receive_stream.h
index fe57643cb0f2d373b37d3b38fb359e1e7c1f373e..babeee0ecb64c4c93d4c961c9c3adaeb100893fb 100644
--- a/webrtc/call/flexfec_receive_stream.h
+++ b/webrtc/call/flexfec_receive_stream.h
@@ -31,8 +31,17 @@ class FlexfecReceiveStream {
   };
 
   struct Config {
+    explicit Config(Transport* rtcp_send_transport)
+        : rtcp_send_transport(rtcp_send_transport) {
+      RTC_DCHECK(rtcp_send_transport);
+    }
+
     std::string ToString() const;
 
+    // Returns true if all RTP information is available in order to
+    // enable receiving FlexFEC.
+    bool IsCompleteAndEnabled() const;
+
     // Payload type for FlexFEC.
     int payload_type = -1;