Use av_rescale_q() for converting FFmpeg timestamps to base::TimeDelta.

media/filters/ffmpeg_audio_decoder.h

Index: media/filters/ffmpeg_audio_decoder.h
diff --git a/media/filters/ffmpeg_audio_decoder.h b/media/filters/ffmpeg_audio_decoder.h
index ffb99f8e523447aab36fb02230d544923304c09f..f30f3309668ddbcd45f5f8295d8918e0fecb731d 100644
--- a/media/filters/ffmpeg_audio_decoder.h
+++ b/media/filters/ffmpeg_audio_decoder.h
@@ -34,6 +34,10 @@ class FFmpegAudioDecoder : public DecoderBase<AudioDecoder, Buffer> {
   FFmpegAudioDecoder();
   virtual ~FFmpegAudioDecoder();
 
+  // Calculates the duration of an audio buffer based on the sample rate,
+  // channels and bits per sample given the size in bytes.
+  base::TimeDelta CalculateDuration(size_t size);
+
   // A FFmpeg defined structure that holds decoder information, this variable
   // is initialized in OnInitialize().
   AVCodecContext* codec_context_;