Add default case to test method.

net/tools/quic/stateless_rejector_test.cc

Index: net/tools/quic/stateless_rejector_test.cc
diff --git a/net/tools/quic/stateless_rejector_test.cc b/net/tools/quic/stateless_rejector_test.cc
index 1b1f3cb0fa92be34756dc7a41e3ad950ef26c05c..5ec2f19104c2f0c51e414edeead6da530eaabf83 100644
--- a/net/tools/quic/stateless_rejector_test.cc
+++ b/net/tools/quic/stateless_rejector_test.cc
@@ -39,6 +39,9 @@ const char* FlagsModeToString(FlagsMode mode) {
       return "CHEAP_DISABLED";
     case BOTH_DISABLED:
       return "BOTH_DISABLED";
+    default:
+      DLOG(FATAL) << "Unexpected FlagsMode";
+      return nullptr;
   }
 }