Fix ChunkDemuxer seek deadlock

media/base/demuxer_stream.h

Index: media/base/demuxer_stream.h
diff --git a/media/base/demuxer_stream.h b/media/base/demuxer_stream.h
index fb0739b4e2a8b8692b6629d7ed63bfb073adcb7d..28acd51da5df7bed04b133adf30019997af772e3 100644
--- a/media/base/demuxer_stream.h
+++ b/media/base/demuxer_stream.h
@@ -27,7 +27,9 @@ class MEDIA_EXPORT DemuxerStream
 
   // Request a buffer to returned via the provided callback.
   //
-  // Buffers will be non-NULL yet may be end of stream buffers.
+  // Non-NULL buffer pointers will contain media data or signal the end of the
+  // stream. A NULL pointer indicates an aborted Read(). This can happen if the
+  // DemuxerStream gets flushed and doesn't have any more data to return.
   typedef base::Callback<void(const scoped_refptr<Buffer>&)> ReadCallback;
   virtual void Read(const ReadCallback& read_callback) = 0;