Add an ONC property for the third-party VPN provider extension ID

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 0c14f23ae7d4969d8fca400677f5e668fdcc4aa8..8945059aa24f46bd0fbab201e2acc340657195c9 100644
--- a/components/onc/onc_constants.cc
+++ b/components/onc/onc_constants.cc
@@ -349,6 +349,10 @@ const char kPassword[] = "Password";
 const char kPasswordAndOTP[] = "PasswordAndOTP";
 }  // openvpn_user_auth_type
 
+namespace third_party_vpn {
+const char kExtensionID[] = "ExtensionID";
+}  // third_party_vpn
+
 namespace verify_x509 {
 const char kName[] = "Name";
 const char kType[] = "Type";