Fix SDP stream ID mismatch issue when a track's stream changes.

webrtc/pc/mediasession.cc

Index: webrtc/pc/mediasession.cc
diff --git a/webrtc/pc/mediasession.cc b/webrtc/pc/mediasession.cc
index dbd32566d523d3d67d8ca3be8fee17e7190fcf9f..69b10452a775a8cccab00b38dafb5e39f4cfa218 100644
--- a/webrtc/pc/mediasession.cc
+++ b/webrtc/pc/mediasession.cc
@@ -448,8 +448,7 @@ static bool AddStreamParams(MediaType media_type,
     if (stream_it->type != media_type)
       continue;  // Wrong media type.
 
-    const StreamParams* param =
-        GetStreamByIds(*current_streams, "", stream_it->id);
+    StreamParams* param = GetStreamByIds(*current_streams, "", stream_it->id);

[pthatcher1, 2017/04/13 21:31:21]

Instead of making this non-const and adding all this functions to return a
non-const, why not just make a new non-const StreamParams from the const one and
then add that to the content_description down below?  

[Taylor Brandstetter, 2017/04/14 15:52:08]

Just because this way is more convenient/efficient, and I don't see any reason
to not return non-const pointers.

     // groupid is empty for StreamParams generated using
     // MediaSessionDescriptionFactory.
     if (!param) {
@@ -500,6 +499,10 @@ static bool AddStreamParams(MediaType media_type,
       // This is necessary so that we can use the CNAME for other media types.
       current_streams->push_back(stream_param);
     } else {
+      // Use existing generated SSRCs/groups, but update the sync_label and
+      // cname if necessary.

[pthatcher1, 2017/04/13 21:31:22]

Can you explain why this is necessary in the comment?

Also, why does the cname come from options instead of stream_it->cname?

[Taylor Brandstetter, 2017/04/14 15:52:08]

Done. And there is no stream_it->cname.

+      param->cname = options.rtcp_cname;
+      param->sync_label = stream_it->sync_label;
       content_description->AddStream(*param);
     }
   }