Add ONC property for EAP-GTC authentication

BUG=486738
[email protected], [email protected]

Review URL: https://codereview.chromium.org/1137633003

Cr-Commit-Position: refs/heads/master@{#329291}
diff --git a/components/onc/onc_constants.h b/components/onc/onc_constants.h
index a28ec83b..b212d31e 100644
--- a/components/onc/onc_constants.h
+++ b/components/onc/onc_constants.h
@@ -297,6 +297,7 @@
 ONC_EXPORT extern const char kEAP_SIM[];
 ONC_EXPORT extern const char kEAP_TLS[];
 ONC_EXPORT extern const char kEAP_TTLS[];
+ONC_EXPORT extern const char kGTC[];
 ONC_EXPORT extern const char kIdentity[];
 ONC_EXPORT extern const char kInner[];
 ONC_EXPORT extern const char kLEAP[];