Make two private QuicSpdyStreamSequencer methods const.

gfe-relnote: n/a, no functional change.
PiperOrigin-RevId: 254999615
Change-Id: I2463678802db7cc9fa450404b1e96d801dfdc501
diff --git a/quic/core/quic_stream_sequencer_buffer.h b/quic/core/quic_stream_sequencer_buffer.h
index 6347b1c..b0478c0 100644
--- a/quic/core/quic_stream_sequencer_buffer.h
+++ b/quic/core/quic_stream_sequencer_buffer.h
@@ -219,10 +219,10 @@
   QuicStreamOffset NextExpectedByte() const;
 
   // Return |gaps_| as a string: [1024, 1500) [1800, 2048)... for debugging.
-  std::string GapsDebugString();
+  std::string GapsDebugString() const;
 
   // Return all received frames as a string in same format as GapsDebugString();
-  std::string ReceivedFramesDebugString();
+  std::string ReceivedFramesDebugString() const;
 
   // The maximum total capacity of this buffer in byte, as constructed.
   const size_t max_buffer_capacity_bytes_;