[CrOS Tether] Add the notion of a tether DeviceState.

chromeos/network/tether_constants.cc

Index: chromeos/network/tether_constants.cc
diff --git a/chromeos/network/tether_constants.cc b/chromeos/network/tether_constants.cc
index 70867b5a178b227582a745be6c847e0c88988e98..72fd96b32a4e44fd63937e324d817ae476db1898 100644
--- a/chromeos/network/tether_constants.cc
+++ b/chromeos/network/tether_constants.cc
@@ -10,5 +10,7 @@ const char kTypeTether[] = "wifi-tether";
 const char kTetherBatteryPercentage[] = "Tether.BatteryPercentage";
 const char kTetherCarrier[] = "Tether.Carrier";
 const char kTetherSignalStrength[] = "Tether.SignalStrength";
+const char kTetherDevicePath[] = "tether-device-path";
+const char kTetherDeviceName[] = "tether-name";
 
 }  // namespace chromeos