Add ONC property for EAP-GTC authentication

components/onc/onc_constants.cc

Index: components/onc/onc_constants.cc
diff --git a/components/onc/onc_constants.cc b/components/onc/onc_constants.cc
index a1cfb2420dcfe333aee6dd219e3278eb91d48148..bb5d61e3ca6ea8534e6b3b7ee920712aa046be60 100644
--- a/components/onc/onc_constants.cc
+++ b/components/onc/onc_constants.cc
@@ -279,6 +279,7 @@ const char kEAP_FAST[] = "EAP-FAST";
 const char kEAP_SIM[] = "EAP-SIM";
 const char kEAP_TLS[] = "EAP-TLS";
 const char kEAP_TTLS[] = "EAP-TTLS";
+const char kGTC[] = "GTC";
 const char kIdentity[] = "Identity";
 const char kInner[] = "Inner";
 const char kLEAP[] = "LEAP";