shill: Implement VPN service disconnect.

BUG=chromium-os:27461,chromium-os:27385
TEST=unit tests

Change-Id: I39c676acf041463a2266c08725f847499e31f5ea
Reviewed-on: https://gerrit.chromium.org/gerrit/17690
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
Commit-Ready: Darin Petkov <petkov@chromium.org>
diff --git a/vpn_service.h b/vpn_service.h
index e0494a5..2a3ada1 100644
--- a/vpn_service.h
+++ b/vpn_service.h
@@ -25,8 +25,10 @@
 
   // Inherited from Service.
   virtual void Connect(Error *error);
+  virtual void Disconnect(Error *error);
   virtual std::string GetStorageIdentifier() const;
-  VPNDriver *driver() { return driver_.get(); }
+
+  VPNDriver *driver() const { return driver_.get(); }
 
  private:
   FRIEND_TEST(VPNServiceTest, GetDeviceRpcId);