Introduce AudioDiscardHelper. Refactor audio decoders to use it.

media/base/audio_buffer.h

Index: media/base/audio_buffer.h
diff --git a/media/base/audio_buffer.h b/media/base/audio_buffer.h
index 902407f4adf66053b690dd23a4ab0c863f53bd82..5d7ab7f5c6700cd05413a691e07408d43177bc53 100644
--- a/media/base/audio_buffer.h
+++ b/media/base/audio_buffer.h
@@ -99,13 +99,8 @@ class MEDIA_EXPORT AudioBuffer
   // Return the channel layout.
   ChannelLayout channel_layout() const { return channel_layout_; }
 
-  // Access to constructor parameters.
   base::TimeDelta timestamp() const { return timestamp_; }
   base::TimeDelta duration() const { return duration_; }
-
-  // TODO(jrummell): Remove set_timestamp() and set_duration() once
-  // DecryptingAudioDecoder::EnqueueFrames() is changed to set them when
-  // creating the buffer. See http://crbug.com/255261.
   void set_timestamp(base::TimeDelta timestamp) { timestamp_ = timestamp; }
   void set_duration(base::TimeDelta duration) { duration_ = duration; }