Bug 1401592: Make sure JSEP distinguishes unicast with rid from unicast without rid. r=mjf

Differential Revision: https://phabricator.services.mozilla.com/D157657

dom/media/webrtc/jsep/JsepTrack.cpp

@@ -229,9 +229,7 @@ void JsepTrack::SendTrackSetRemote(SsrcGenerator& aSsrcGenerator,
     mRids = std::move(newRids);
   }
 
-  // Treat "simulcast" with one rid as unicast
-  if (mRids.size() <= 1) {
-    mRids.clear();
+  if (mRids.empty()) {
     mRids.push_back("");
   }
 

dom/media/webrtc/jsep/JsepTrack.h

@@ -271,8 +271,11 @@ class JsepTrack {
   sdp::Direction mDirection;
   std::vector<UniquePtr<JsepCodecDescription>> mPrototypeCodecs;
   // List of rids. May be initially populated from JS, or from a remote SDP.
-  // Can be updated by remote SDP. For unicast, this will contain one element
-  // with the empty string.
+  // Can be updated by remote SDP. If no negotiation has taken place at all,
+  // this will be empty. If negotiation has taken place, but no simulcast
+  // attr was negotiated, this will contain the empty string as a single
+  // element. If a simulcast attribute was negotiated, this will contain the
+  // negotiated rids.
   std::vector<std::string> mRids;
   UniquePtr<JsepTrackNegotiatedDetails> mNegotiatedDetails;
   std::vector<uint32_t> mSsrcs;