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/cellular.cc b/cellular.cc
index 2c56b9d..f1d7a45 100644
--- a/cellular.cc
+++ b/cellular.cc
@@ -372,7 +372,7 @@
   // TODO(petkov): Set old_url/usage_url.
 }
 
-bool Cellular::TechnologyIs(const Device::Technology type) {
+bool Cellular::TechnologyIs(const Device::Technology type) const {
   return type == Device::kCellular;
 }