Add a missing case in SerializedPacketFateToString for DISCARD.

PiperOrigin-RevId: 433769557
diff --git a/quic/core/quic_types.cc b/quic/core/quic_types.cc
index 70c9101..7120670 100644
--- a/quic/core/quic_types.cc
+++ b/quic/core/quic_types.cc
@@ -296,13 +296,13 @@
 
 std::string SerializedPacketFateToString(SerializedPacketFate fate) {
   switch (fate) {
+    RETURN_STRING_LITERAL(DISCARD);
     RETURN_STRING_LITERAL(COALESCE);
     RETURN_STRING_LITERAL(BUFFER);
     RETURN_STRING_LITERAL(SEND_TO_WRITER);
     RETURN_STRING_LITERAL(LEGACY_VERSION_ENCAPSULATE);
-    default:
-      return absl::StrCat("Unknown(", static_cast<int>(fate), ")");
   }
+  return absl::StrCat("Unknown(", static_cast<int>(fate), ")");
 }
 
 std::ostream& operator<<(std::ostream& os, SerializedPacketFate fate) {