shill: vpn: Make L2TPIPSec driver use the base VPNDriver's KeyValueStore.

This should fix property handling which was currently split between
the base and the derived class.

BUG=chromium-os:30045
TEST=unit tests

Change-Id: I74268b3825223907d483bb82539788a29ff05d78
Reviewed-on: https://gerrit.chromium.org/gerrit/21182
Commit-Ready: Darin Petkov <petkov@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
diff --git a/l2tp_ipsec_driver_unittest.cc b/l2tp_ipsec_driver_unittest.cc
index a1ccb1b..02008d3 100644
--- a/l2tp_ipsec_driver_unittest.cc
+++ b/l2tp_ipsec_driver_unittest.cc
@@ -69,7 +69,7 @@
   static const int kInterfaceIndex;
 
   void SetArg(const string &arg, const string &value) {
-    driver_->args_.SetString(arg, value);
+    driver_->args()->SetString(arg, value);
   }
 
   KeyValueStore *GetArgs() {