shill: LinkEvent shouldn't call DestroyService

In Cellular::LinkEvent, if shill sees that the network interface has
gone down, it now calls DropConnection instead of calling DestroyService.
In addition, shill no longer takes down the network interface in
Cellular::DisconnectCleanup.

BUG=chromium-os:36573
TEST=unit tests and network_3GModemControl

Change-Id: Id64a3cc0245cfb75cfbd68529473683a7313af22
Reviewed-on: https://gerrit.chromium.org/gerrit/38756
Commit-Ready: Arman Uguray <armansito@chromium.org>
Reviewed-by: Arman Uguray <armansito@chromium.org>
Tested-by: Arman Uguray <armansito@chromium.org>
diff --git a/manager.h b/manager.h
index e1f9c67..50e9f0d 100644
--- a/manager.h
+++ b/manager.h
@@ -269,6 +269,7 @@
   friend class WiMaxProviderTest;
 
   FRIEND_TEST(CellularTest, ConnectAddsTerminationAction);
+  FRIEND_TEST(CellularTest, LinkEventWontDestroyService);
   FRIEND_TEST(ManagerTest, AvailableTechnologies);
   FRIEND_TEST(ManagerTest, CalculateStateOffline);
   FRIEND_TEST(ManagerTest, CalculateStateOnline);