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

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..04dca38f562558dbc92c8c3cb33a170bcb07346c 100644
--- a/media/base/audio_buffer.h
+++ b/media/base/audio_buffer.h
@@ -87,6 +87,11 @@ class MEDIA_EXPORT AudioBuffer
   // Duration is adjusted to reflect the fewer frames.
   void TrimEnd(int frames_to_trim);
 
+  // Trim an AudioBuffer by removing |end - start| frames starting at |start|
+  // and ending at |end|; i.e. exclusive.  Even if start is zero, timestamp() is
+  // not adjusted, only duration().
+  void TrimRange(int start, int end);
+
   // Return the number of channels.
   int channel_count() const { return channel_count_; }