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.cc b/quic/core/quic_stream_sequencer_buffer.cc
index f4ff276..e68925b 100644
--- a/quic/core/quic_stream_sequencer_buffer.cc
+++ b/quic/core/quic_stream_sequencer_buffer.cc
@@ -500,12 +500,12 @@
   }
 }
 
-std::string QuicStreamSequencerBuffer::GapsDebugString() {
+std::string QuicStreamSequencerBuffer::GapsDebugString() const {
   // TODO(vasilvv): this should return the complement of |bytes_received_|.
   return bytes_received_.ToString();
 }
 
-std::string QuicStreamSequencerBuffer::ReceivedFramesDebugString() {
+std::string QuicStreamSequencerBuffer::ReceivedFramesDebugString() const {
   return bytes_received_.ToString();
 }