Be less pessimistic about turning "default" receive streams into signaled streams.

webrtc/media/engine/webrtcvoiceengine.cc

Index: webrtc/media/engine/webrtcvoiceengine.cc
diff --git a/webrtc/media/engine/webrtcvoiceengine.cc b/webrtc/media/engine/webrtcvoiceengine.cc
index 2db552611220775e67f4f82a936ec949d2b01934..eb9ab71762a42d800a59ff2bbce193c0e9993f29 100644
--- a/webrtc/media/engine/webrtcvoiceengine.cc
+++ b/webrtc/media/engine/webrtcvoiceengine.cc
@@ -2272,10 +2272,11 @@ bool WebRtcVoiceMediaChannel::AddRecvStream(const StreamParams& sp) {
     return false;
   }
 
-  // Remove the default receive stream if one had been created with this ssrc;
-  // we'll recreate it then.
+  // If the default receive stream was created with this ssrc, just unmark it as
+  // being the default stream.
   if (IsDefaultRecvStream(ssrc)) {
-    RemoveRecvStream(ssrc);
+    default_recv_ssrc_ = -1;

[Taylor Brandstetter, 2017/02/08 17:49:33]

If you do this, does the default stream get sp.sync_label and end up synced with
video?

+    return true;
   }
 
   if (GetReceiveChannelId(ssrc) != -1) {