shill: cellular: Add Network.Shill.Cellular.DropsPerHour.

BUG=chromium-os:38557
TEST=Unit tests, manually lose LTE signal and check chrome://histograms

Change-Id: I82aa1edabd21c5dfde4834cd36a34a12b629ba8f
Reviewed-on: https://gerrit.chromium.org/gerrit/42813
Reviewed-by: Arman Uguray <armansito@chromium.org>
Commit-Queue: Thieu Le <thieule@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>
diff --git a/manager.h b/manager.h
index d4989bd..5d9a32f 100644
--- a/manager.h
+++ b/manager.h
@@ -83,7 +83,7 @@
   void AddDeviceToBlackList(const std::string &device_name);
 
   virtual void Start();
-  void Stop();
+  virtual void Stop();
   bool running() const { return running_; }
 
   const ProfileRefPtr &ActiveProfile() const;