Refactor FFmpegVideoDecoder to try and generalize code common to all video decoders.

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 f7adc3336febf46c98e45ed28280671e99d5d552..f5ff8ac5bc3e9568b7248e71f83e7cad6fdfe6cc 100644
--- a/media/filters/ffmpeg_audio_decoder.h
+++ b/media/filters/ffmpeg_audio_decoder.h
@@ -24,13 +24,12 @@ class FFmpegAudioDecoder : public DecoderBase<AudioDecoder, Buffer> {
   static bool IsMediaFormatSupported(const MediaFormat& media_format);
 
  protected:
-  virtual bool OnInitialize(DemuxerStream* demuxer_stream);
+  virtual void DoInitialize(DemuxerStream* demuxer_stream, bool* success,
+                            Task* done_cb);
 
-  virtual void OnSeek(base::TimeDelta time);
+  virtual void DoSeek(base::TimeDelta time, Task* done_cb);
 
-  virtual void OnStop();
-
-  virtual void OnDecode(Buffer* input);
+  virtual void DoDecode(Buffer* input, Task* done_cb);
 
  private:
   friend class FilterFactoryImpl0<FFmpegAudioDecoder>;