Linux: make it possible to compile against libav when use_system_ffmpeg==1

media/filters/ffmpeg_audio_decoder.cc

Index: media/filters/ffmpeg_audio_decoder.cc
diff --git a/media/filters/ffmpeg_audio_decoder.cc b/media/filters/ffmpeg_audio_decoder.cc
index 6df6112e7526dfbefa18044b643484fe8a51862e..4727091503a7110aab87a3ed711c2f7465ff02c7 100644
--- a/media/filters/ffmpeg_audio_decoder.cc
+++ b/media/filters/ffmpeg_audio_decoder.cc
@@ -398,12 +398,12 @@ void FFmpegAudioDecoder::RunDecodeLoop(
     int decoded_audio_size = 0;
     if (frame_decoded) {
       if (av_frame_->sample_rate != samples_per_second_ ||
-          av_frame_->channels != channels_ ||
+          av_frame_get_channels(av_frame_) != channels_ ||
           av_frame_->format != av_sample_format_) {
         DLOG(ERROR) << "Unsupported midstream configuration change!"
                     << " Sample Rate: " << av_frame_->sample_rate << " vs "
                     << samples_per_second_
-                    << ", Channels: " << av_frame_->channels << " vs "
+                    << ", Channels: " << av_frame_get_channels(av_frame_) << " vs "
                     << channels_
                     << ", Sample Format: " << av_frame_->format << " vs "
                     << av_sample_format_;