Enable passing receive stream ID to the decoder factory.

webrtc/media/engine/webrtcvideoengine2.cc

Index: webrtc/media/engine/webrtcvideoengine2.cc
diff --git a/webrtc/media/engine/webrtcvideoengine2.cc b/webrtc/media/engine/webrtcvideoengine2.cc
index f32749199f90e6d67218c381beedb3ef30174931..225ac80584c7521917083ec0760eefecd8e94448 100644
--- a/webrtc/media/engine/webrtcvideoengine2.cc
+++ b/webrtc/media/engine/webrtcvideoengine2.cc
@@ -2252,6 +2252,7 @@ WebRtcVideoChannel2::WebRtcVideoReceiveStream::GetSsrcs() const {
 
 WebRtcVideoChannel2::WebRtcVideoReceiveStream::AllocatedDecoder
 WebRtcVideoChannel2::WebRtcVideoReceiveStream::CreateOrReuseVideoDecoder(
+    uint32_t ssrc,
     std::vector<AllocatedDecoder>* old_decoders,
     const VideoCodec& codec) {
   webrtc::VideoCodecType type = CodecTypeFromName(codec.name);
@@ -2267,7 +2268,7 @@ WebRtcVideoChannel2::WebRtcVideoReceiveStream::CreateOrReuseVideoDecoder(
 
   if (external_decoder_factory_ != NULL) {
     webrtc::VideoDecoder* decoder =
-        external_decoder_factory_->CreateVideoDecoder(type);
+        external_decoder_factory_->CreateVideoDecoderForSsrc(ssrc, type);

[pthatcher1, 2016/06/14 21:12:04]

Here's a possible solution:  

1.  The WebRtcVideoChannel2::WebRtcVideoReceiveStream is passed an ID, which is
the same as the RtpReceiver's id() (and in the future, MID).  But the
WebRtcVideoReceiveStream doesn't care.  It's just an ID.

2.  That id() is passed into CreateVideoDecoder here as CreateVideoDecoder(type,
{id()}), where hopefully the last thing is a struct with a .receive_stream_id.

3.  We expose the SSRCs in the RtpReceiver::GetParameters.

4.  By enumerating over the PeerConnection's RtpReceivers, you can get the
mapping of ssrc <-> stream_id and thus the decoder can know the ssrc from the
.stream_id.

Just an idea, but it wouldn't be very hard, and it wouldn't be too hacky.

     if (decoder != NULL) {
       return AllocatedDecoder(decoder, type, true);
     }
@@ -2300,8 +2301,9 @@ void WebRtcVideoChannel2::WebRtcVideoReceiveStream::ConfigureCodecs(
   allocated_decoders_.clear();
   config_.decoders.clear();
   for (size_t i = 0; i < recv_codecs.size(); ++i) {
+    uint32_t ssrc = ssrcs_.empty() ? 0 : ssrcs_[0];
     AllocatedDecoder allocated_decoder =
-        CreateOrReuseVideoDecoder(old_decoders, recv_codecs[i].codec);
+        CreateOrReuseVideoDecoder(ssrc, old_decoders, recv_codecs[i].codec);
     allocated_decoders_.push_back(allocated_decoder);
 
     webrtc::VideoReceiveStream::Decoder decoder;