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

chromeos/network/device_state.h

Index: chromeos/network/device_state.h
diff --git a/chromeos/network/device_state.h b/chromeos/network/device_state.h
index 97ebd0b908d2c7fdea32ea45e72235d7f2af121a..59bd3d82a61772f340e15bfb997d6dbfb0e64147 100644
--- a/chromeos/network/device_state.h
+++ b/chromeos/network/device_state.h
@@ -35,6 +35,7 @@ class CHROMEOS_EXPORT DeviceState : public ManagedState {
   // Accessors
   const std::string& mac_address() const { return mac_address_; }
   bool scanning() const { return scanning_; }
+  void set_scanning(bool scanning) { scanning_ = scanning; }
 
   // Cellular specific accessors
   const std::string& home_provider_id() const { return home_provider_id_; }