Add an ONC property for setting per-network roam threshold

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 6e1684535dcf6444d6774ca821a51625f2603255..7183369ea078b358187179a784af5ef5f245a3c0 100644
--- a/components/onc/onc_constants.cc
+++ b/components/onc/onc_constants.cc
@@ -201,6 +201,7 @@ const char kFrequencyList[] = "FrequencyList";
 const char kHexSSID[] = "HexSSID";
 const char kHiddenSSID[] = "HiddenSSID";
 const char kPassphrase[] = "Passphrase";
+const char kRoamThreshold[] = "RoamThreshold";
 const char kSSID[] = "SSID";
 const char kSecurity[] = "Security";
 const char kSecurityNone[] = "None";