shill: vpn: Return service from VPNProvider's service_ list

BUG=chromium-os:28157
TEST=New unit test

Change-Id: Id2f1f5f336fd47d3d0c769789151386306c21c0a
Reviewed-on: https://gerrit.chromium.org/gerrit/19844
Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/openvpn_driver.h b/openvpn_driver.h
index c325b10..d5cedb5 100644
--- a/openvpn_driver.h
+++ b/openvpn_driver.h
@@ -62,6 +62,9 @@
   virtual void OnReconnecting();
 
   virtual void InitPropertyStore(PropertyStore *store);
+
+  virtual std::string GetProviderType() const;
+
   void ClearMappedProperty(const size_t &index, Error *error);
   std::string GetMappedProperty(const size_t &index, Error *error);
   void SetMappedProperty(const size_t &index,