Allow default audio receive channel to receive on any unsignalled SSRC.

talk/media/webrtc/webrtcvoiceengine.cc

Index: talk/media/webrtc/webrtcvoiceengine.cc
diff --git a/talk/media/webrtc/webrtcvoiceengine.cc b/talk/media/webrtc/webrtcvoiceengine.cc
index c13b22933a323f99eba553d5b84229cf992417b3..d124305cd092f04306e8d6b7b36725f8b81dbe15 100644
--- a/talk/media/webrtc/webrtcvoiceengine.cc
+++ b/talk/media/webrtc/webrtcvoiceengine.cc
@@ -2445,8 +2445,9 @@ void WebRtcVoiceMediaChannel::OnPacketReceived(
     return;
   }
 
-  if (receive_channels_.empty()) {
-    // Create new channel, which will be the default receive channel.
+  // If we don't have a default channel, and the SSRC is unknown, create a
+  // default channel.
+  if (default_recv_ssrc_ == -1 && GetReceiveChannelId(ssrc) == -1) {

[pthatcher1, 2015/11/19 17:16:57]

This might be more clear as:

bool known_ssrc = (GetReceiveChannelId(ssrc) >= 0);
bool have_default_recv_channel = (default_recv_ssrc_ >= 0);

if (!known_ssrc && !have_default_recv_channel) {
  // Create a default recv channel
  ... call to AddRecvStream ...
}

// If the SSRC is unknown here, route it to the default channel.
// See: https://bugs.chromium.org/p/webrtc/issues/detail?id=5208
int channel = GetReceiveChannelId(known_ssrc ? ssrc : default_recv_ssrc_);
RTC_DCHECK(channel != -1);

... call to call_->Receiver()->DeliverPacket ...

if (known_ssrc && webrtc::PacketReceiver::DELIVERY_OK != delivery_result) {
  return;
}

... call to engine()->voe()->network()->ReceivedRTPPacket ...

[the sun, 2015/11/19 20:33:22]

I tried this, mainly to get rid of the extra map lookup (which both mine and
your solution requires), but I felt the whole thing turning into an
uncomfortably large change to downstream. So if it is ok with you I'd rather
keep it this simple and try to land the changes required to move packet delivery
in under webrtc::AudioReceiveStream for the next cut.

[pthatcher1, 2015/11/19 21:01:22]

So you're suggesting:

1.  Land the minimal diff that changes behavior and then pull over to chromium.
2.  Land a "make it more readable" CL that doesn't change behavior.

That sounds good to me, as long as we remember to do #2.  

     StreamParams sp;
     sp.ssrcs.push_back(ssrc);
     LOG(LS_INFO) << "Creating default receive stream for SSRC=" << ssrc << ".";
@@ -2466,7 +2467,13 @@ void WebRtcVoiceMediaChannel::OnPacketReceived(
           reinterpret_cast<const uint8_t*>(packet->data()), packet->size(),
           webrtc_packet_time);
   if (webrtc::PacketReceiver::DELIVERY_OK != delivery_result) {
-    return;
+    // If the SSRC is unknown here, route it to the default channel, if we have
+    // one. See: https://bugs.chromium.org/p/webrtc/issues/detail?id=5208
+    if (default_recv_ssrc_ == -1) {
+      return;
+    } else {
+      ssrc = default_recv_ssrc_;
+    }
   }
 
   // Find the channel to send this packet to. It must exist since webrtc::Call