Adds a DebugString method to QuicStreamSequencer.

net/quic/quic_stream_sequencer.h

Index: net/quic/quic_stream_sequencer.h
diff --git a/net/quic/quic_stream_sequencer.h b/net/quic/quic_stream_sequencer.h
index 3f37a7ec35fbe66ee009dec11f088d814445a690..d0e5d93a002060a79da90baceac18d63e34ac17c 100644
--- a/net/quic/quic_stream_sequencer.h
+++ b/net/quic/quic_stream_sequencer.h
@@ -95,6 +95,9 @@ class NET_EXPORT_PRIVATE QuicStreamSequencer {
 
   bool ignore_read_data() const { return ignore_read_data_; }
 
+  // Returns std::string describing internal state.
+  const std::string DebugString() const;
+
  private:
   friend class test::QuicStreamSequencerPeer;