Add optional stereo codec to SDP negotiation

- Defines stereo codec case, similar to RTX, that adds stereo codec to the SDP
negotiation. The underlying codec's payload type is similarly defined by "apt".
- If this negotiation is successful, codec name is included in sdp line via
"acn".
- Adds codec setting initializers for these specific stereo cases.
- Introduces new Stereo*Factory classes as optional convenience wrappers that
inserts stereo codec to the existing set of supported codecs on demand.

This CL is the step 5 for adding alpha channel support over the wire in webrtc.
Design Doc: https://goo.gl/sFeSUT

Bug: webrtc:7671
Change-Id: Ie12c56c8fcf7934e216135d73af33adec5248f76
Reviewed-on: https://webrtc-review.googlesource.com/22901
Commit-Queue: Niklas Enbom <niklas.enbom@webrtc.org>
Reviewed-by: Niklas Enbom <niklas.enbom@webrtc.org>
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21210}

modules/video_coding/video_codec_initializer.cc

@@ -39,6 +39,20 @@ bool VideoCodecInitializer::SetupCodec(
     bool nack_enabled,
     VideoCodec* codec,
     std::unique_ptr<VideoBitrateAllocator>* bitrate_allocator) {
+  if (PayloadStringToCodecType(settings.payload_name) == kVideoCodecStereo) {
+    VideoSendStream::Config::EncoderSettings associated_codec_settings =
+        settings;
+    associated_codec_settings.payload_name =
+        CodecTypeToPayloadString(kVideoCodecVP9);
+    if (!SetupCodec(config, associated_codec_settings, streams, nack_enabled,
+                    codec, bitrate_allocator)) {
+      RTC_LOG(LS_ERROR) << "Failed to create stereo encoder configuration.";
+      return false;
+    }
+    codec->codecType = kVideoCodecStereo;
+    return true;
+  }
+
   *codec =
       VideoEncoderConfigToVideoCodec(config, streams, settings.payload_name,
                                      settings.payload_type, nack_enabled);