Discard old packets from the QuicSentPacketManager which are only

net/quic/quic_unacked_packet_map.h

Index: net/quic/quic_unacked_packet_map.h
diff --git a/net/quic/quic_unacked_packet_map.h b/net/quic/quic_unacked_packet_map.h
index 293c7b6a16586adf715e9addec43ad505a45eafa..a7a0a6cc54264ec5207bf77f84b18c98623b1926 100644
--- a/net/quic/quic_unacked_packet_map.h
+++ b/net/quic/quic_unacked_packet_map.h
@@ -139,6 +139,10 @@ class NET_EXPORT_PRIVATE QuicUnackedPacketMap {
   // Returns true if the packet no longer has a purpose in the map.
   bool IsPacketUseless(QuicPacketSequenceNumber sequence_number,
                        const TransmissionInfo& info) const;
+  // Returns true if the packet is useless or it's only purpose is RTT
+  // measurement, and it's old enough that is unlikely to ever happen.
+  bool IsPacketRemovable(QuicPacketSequenceNumber sequence_number,
+                         const TransmissionInfo& info) const;
 
   QuicPacketSequenceNumber largest_sent_packet_;
   QuicPacketSequenceNumber largest_observed_;