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 1591 matching lines @@
         if (it == decoder_map.end() || *it != item) {
           return false;  // The old map isn't a subset of the new map.
         }
       }
       return true;
     }());
     config_.decoder_map = decoder_map;
     RecreateAudioReceiveStream();
   }
 
+  void MaybeRecreateAudioReceiveStream(const std::string& sync_group) {
+    RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
+    if (config_.sync_group != sync_group) {
+      config_.sync_group = sync_group;
+      RecreateAudioReceiveStream();
+    }
+  }
+
   webrtc::AudioReceiveStream::Stats GetStats() const {
     RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
     RTC_DCHECK(stream_);
     return stream_->GetStats();
   }
 
   int channel() const {
     RTC_DCHECK(worker_thread_checker_.CalledOnValidThread());
     return config_.voe_channel_id;
   }
@@ skipping 647 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, we unmark it as
+  // being the default stream, and possibly recreate the AudioReceiveStream, if
+  // sync_label has changed.
   if (IsDefaultRecvStream(ssrc)) {
-    RemoveRecvStream(ssrc);
+    recv_streams_[ssrc]->MaybeRecreateAudioReceiveStream(sp.sync_label);
+    default_recv_ssrc_ = -1;
+    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 405 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