shill: Add Cellular devices to DeviceInfo.

This allows DeviceInfo to handle link messages for Cellular devices, so that we
can bring their interfaces up and down.

BUG=chromium-os:18438,18855
TEST=unit tests, tested on device

Change-Id: I7d642c7302472472c56042f731f746c8b997a149
Reviewed-on: http://gerrit.chromium.org/gerrit/5604
Tested-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
diff --git a/wifi.h b/wifi.h
index 2808b79..86969da 100644
--- a/wifi.h
+++ b/wifi.h
@@ -32,7 +32,7 @@
   virtual ~WiFi();
   virtual void Start();
   virtual void Stop();
-  virtual bool TechnologyIs(const Technology type);
+  virtual bool TechnologyIs(const Technology type) const;
 
   // called by SupplicantInterfaceProxy, in response to events from
   // wpa_supplicant.