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

chromeos/test/data/network/invalid_settings_with_repairs.json

 {
     "duplicate-network-guid": {
           "NetworkConfigurations": [
              {
                "GUID": "1",
                 "Type": "WiFi",
                 "Name": "My WiFi Network",
                 "WiFi": {
                   "Passphrase": "dummy",
                   "SSID": "dummySSID",
@@ skipping 378 matching lines @@
         "Host": "host",
         "Type": "OpenVPN",
         "OpenVPN": {
           "ClientCertType": "None",
           "VerifyX509": {
             "Type": "unknown type"
           }
         }
       }
     },
+    "third-party-vpn-missing-extension-id": {
+      "GUID": "guid",
+      "Name": "third-party VPN",
+      "Type": "VPN",
+      "VPN": {
+        "Type": "ThirdPartyVPN",
+        "ThirdPartyVPN": { }
+      }
+    },
     "toplevel-empty": {
         "Type": "UnencryptedConfiguration",
         "NetworkConfigurations": [ ]
     },
     "toplevel-repaired": {
         "Type": "UnencryptedConfiguration",
         "NetworkConfigurations":
         [ { "GUID": "guid",
             "Type": "Ethernet",
             "Name": "eth0",
@@ skipping 113 matching lines @@
             "SSID": "ssid-wpa",
             "UnknownField1": "Value1",
             "Security": "WPA-PSK" }
         }
       ],
       "Certificates": [],
       "Type": "UnencryptedConfiguration",
       "UnknownField3": [],
     }
 }