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

webrtc/media/engine/webrtcvoiceengine.cc

 /*
  *  Copyright (c) 2004 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 2254 matching lines @@
   if (!ValidateStreamParams(sp)) {
     return false;
   }
 
   const uint32_t ssrc = sp.first_ssrc();
   if (ssrc == 0) {
     LOG(LS_WARNING) << "AddRecvStream with ssrc==0 is not supported.";
     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) {
     LOG(LS_ERROR) << "Stream already exists with ssrc " << ssrc;
     return false;
   }
 
   // Create a new channel for receiving audio data.
   const int channel = CreateVoEChannel();
   if (channel == -1) {
@@ skipping 430 matching lines @@
   RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
   const auto it = send_streams_.find(ssrc);
   if (it != send_streams_.end()) {
     return it->second->channel();
   }
   return -1;
 }
 }  // namespace cricket
 
 #endif  // HAVE_WEBRTC_VOICE