Prefer external video codecs over internal in SDP

webrtc/media/engine/webrtcvideoengine.cc

 /*
  *  Copyright (c) 2014 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 519 matching lines @@
 }
 
 static std::vector<VideoCodec> GetSupportedCodecs(
     const WebRtcVideoEncoderFactory* external_encoder_factory) {
   const std::vector<VideoCodec> internal_codecs =
       InternalEncoderFactory().supported_codecs();
   LOG(LS_INFO) << "Internally supported codecs: "
                << CodecVectorToString(internal_codecs);
 
   std::vector<VideoCodec> unified_codecs;
-  AppendVideoCodecs(internal_codecs, &unified_codecs);
-
   if (external_encoder_factory != nullptr) {
     const std::vector<VideoCodec>& external_codecs =
         external_encoder_factory->supported_codecs();
     AppendVideoCodecs(external_codecs, &unified_codecs);
     LOG(LS_INFO) << "Codecs supported by the external encoder factory: "
                  << CodecVectorToString(external_codecs);
   }
 
+  AppendVideoCodecs(internal_codecs, &unified_codecs);
+
   return unified_codecs;
 }
 
 WebRtcVideoChannel::WebRtcVideoChannel(
     webrtc::Call* call,
     const MediaConfig& config,
     const VideoOptions& options,
     WebRtcVideoEncoderFactory* external_encoder_factory,
     WebRtcVideoDecoderFactory* external_decoder_factory)
     : VideoMediaChannel(config),
@@ skipping 2081 matching lines @@
     stream.temporal_layer_thresholds_bps.resize(GetDefaultVp9TemporalLayers() -
                                                 1);
   }
 
   std::vector<webrtc::VideoStream> streams;
   streams.push_back(stream);
   return streams;
 }
 
 }  // namespace cricket