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 2401ed81dd5bdb85a3763b93d9397f1074475be6..a6a0ddc278c9e9bad1f4b7fe1105f9993ebac0c8 100644
--- a/media/base/audio_buffer.h
+++ b/media/base/audio_buffer.h
@@ -85,6 +85,10 @@ 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 from [|start|, |end|).
+  // 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_; }