shill: Destroy cellular connection when downgrading from registered state.

BUG=chromium-os:24953
TEST=tested on device

Change-Id: I050a3702aa91a90930b457b87ec4ce6bc5105517
Reviewed-on: https://gerrit.chromium.org/gerrit/13975
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
Commit-Ready: Darin Petkov <petkov@chromium.org>
diff --git a/cellular.h b/cellular.h
index 15e9d9f..7551ad0 100644
--- a/cellular.h
+++ b/cellular.h
@@ -209,6 +209,7 @@
   void HandleNewRegistrationStateTask();
 
   void CreateService();
+  void DestroyService();
 
   // Signal callbacks inherited from ModemProxyDelegate.
   virtual void OnModemStateChanged(uint32 old_state,