shill: teach dbus_adaptor (and appropriate subclasses) how
to clear properties

BUG=chromium-os:24814
TEST=new unit tests

Change-Id: Ifad5ad0ad6c00d906bade7e1293540a58ac15625
Reviewed-on: https://gerrit.chromium.org/gerrit/15291
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: mukesh agrawal <quiche@chromium.org>
Commit-Ready: mukesh agrawal <quiche@chromium.org>
diff --git a/wifi.h b/wifi.h
index cdfb30c..442e93d 100644
--- a/wifi.h
+++ b/wifi.h
@@ -68,6 +68,7 @@
   friend class WiFiMainTest;  // access to supplicant_*_proxy_, link_up_
   FRIEND_TEST(WiFiMainTest, InitialSupplicantState);  // kInterfaceStateUnknown
   FRIEND_TEST(WiFiMainTest, ScanResultsWithUpdates);  // EndpointMap
+  FRIEND_TEST(WiFiPropertyTest, ClearDerivedProperty);  // bgscan_method_
 
   typedef std::map<const std::string, WiFiEndpointRefPtr> EndpointMap;
   typedef std::map<WiFiService *, std::string> ReverseServiceMap;