Cleanup OPUS decoder. Remove extraneous transforms and copies.

media/ffmpeg/ffmpeg_common.cc

Index: media/ffmpeg/ffmpeg_common.cc
diff --git a/media/ffmpeg/ffmpeg_common.cc b/media/ffmpeg/ffmpeg_common.cc
index 40696c690998ff2ea918010d092d614c35e2e63a..ff474e909f5e70f0c22610a534354725603684a5 100644
--- a/media/ffmpeg/ffmpeg_common.cc
+++ b/media/ffmpeg/ffmpeg_common.cc
@@ -286,7 +286,8 @@ static void AVCodecContextToAudioDecoderConfig(
 
   if (codec == kCodecOpus) {
     // |codec_context->sample_fmt| is not set by FFmpeg because Opus decoding is
-    // not enabled in FFmpeg, so we need to manually set the sample format.
+    // not enabled in FFmpeg.  It doesn't matter what value is set here, so long
+    // as it's valid, the true sample format is selected inside the decoder.
     sample_format = kSampleFormatS16;
   }