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_unittest.cc

@@ -79,6 +79,7 @@ class VideoCodecInitializerTest : public ::testing::Test {
           webrtc::VideoEncoderConfig::Vp8EncoderSpecificSettings>(vp8_settings);
       settings_.payload_name = kVp8PayloadName;
       settings_.payload_type = kVp8PayloadType;
+    } else if (type == VideoCodecType::kVideoCodecStereo) {
     } else {
       ADD_FAILURE() << "Unexpected codec type: " << type;
     }
@@ -93,7 +94,8 @@ class VideoCodecInitializerTest : public ::testing::Test {
                                            &bitrate_allocator_out_)) {
       return false;
     }
-
+    if (codec_out_.codecType == VideoCodecType::kVideoCodecStereo)
+      return true;
     // Make sure temporal layers instances have been created.
     if (codec_out_.codecType == VideoCodecType::kVideoCodecVP8) {
       if (!codec_out_.VP8()->tl_factory)
@@ -215,4 +217,10 @@ TEST_F(VideoCodecInitializerTest, HighFpsSimlucastVp8Screenshare) {
             bitrate_allocation.GetBitrate(1, 1));
 }
 
+TEST_F(VideoCodecInitializerTest, SingleStreamStereoCodec) {
+  SetUpFor(VideoCodecType::kVideoCodecStereo, 1, 1, true);
+  streams_.push_back(DefaultStream());
+  EXPECT_TRUE(InitializeCodec());
+}
+
 }  // namespace webrtc