Support start trimming post-decoding. Use it with FFmpegDemuxer.

media/base/audio_discard_helper.h

Index: media/base/audio_discard_helper.h
diff --git a/media/base/audio_discard_helper.h b/media/base/audio_discard_helper.h
index 6a49239381bf4be7b7eaf5186cd55d7629efb336..21b16dc9175d0775be8a3b415b3801d44074c91c 100644
--- a/media/base/audio_discard_helper.h
+++ b/media/base/audio_discard_helper.h
@@ -19,7 +19,13 @@ class DecoderBuffer;
 // Helper class for managing timestamps and discard events around decoding.
 class MEDIA_EXPORT AudioDiscardHelper {
  public:
-  explicit AudioDiscardHelper(int sample_rate);
+  // |sample_rate| is the sample rate of decode data which will be handed into
+  // the ProcessBuffers() call.
+  //
+  // |codec_delay| is used to figure out the the start of decoded data from each
+  // corresponding encoded buffer.  End discard is only supported when there is
+  // no |codec_delay|.
+  AudioDiscardHelper(int sample_rate, size_t codec_delay);
   ~AudioDiscardHelper();
 
   // Converts a TimeDelta to a frame count based on the constructed sample rate.
@@ -43,10 +49,12 @@ class MEDIA_EXPORT AudioDiscardHelper {
 
  private:
   const int sample_rate_;
+  const size_t codec_delay_;
   AudioTimestampHelper timestamp_helper_;
 
   size_t discard_frames_;
   base::TimeDelta last_input_timestamp_;
+  scoped_refptr<DecoderBuffer> delayed_discard_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(AudioDiscardHelper);
 };