Add an ONC property for proactive key caching.

chromeos/test/data/network/wifi_clientcert_with_cert_pems.onc

 {
     "GUID": "{77db0089-0bc8-4358-929c-123xcv}",
     "Type": "WiFi",
     "Name": "SomeWifi-XY",
     "WiFi": {
         "HexSSID": "4F70656E577274", // Hex of "OpenWrt"
         "Security": "WPA-EAP",
         "Recommended": [ "AutoConnect" ],
         "EAP": {
             "Outer": "EAP-TLS",
             "Identity": "abc ${LOGIN_ID}@my.domain.com",
+            "UseProactiveKeyCaching": true,
             "UseSystemCAs": true,
             "ClientCertType": "Pattern",
             "ClientCertPattern": {
                 "IssuerCAPEMs": [
                     "pem1",
                     "pem2"
                 ],
                 "EnrollmentURI": [ "chrome-extension://delkjfjibodjclmdijflfnimdmgdagfk/generate-cert.html" ]
             },
             "Recommended": [ "Identity" ],
             "SaveCredentials": true
         }
     },
     "ProxySettings": {
         "Type": "PAC",
         "PAC": "https://proxycfg.my.domain.com/proxy.dat"
     }
 }