Skip invalid DefaultService object path values.

If shill sends an invalid DefaultService object path value, we need to
explicitly treat since otherwise the DBus library will abort the
program when trying to send a message to the invalid object path.

Bug: chromium:526446
Change-Id: Id91787916b62cd94dab38532b98be0f0a8b6d4c4
Test: Added unittests
diff --git a/fake_shill_proxy.h b/fake_shill_proxy.h
index 8daf7c2..ae17eaa 100644
--- a/fake_shill_proxy.h
+++ b/fake_shill_proxy.h
@@ -42,11 +42,11 @@
   // with SetServiceForPath().
   org::chromium::flimflam::ManagerProxyMock* GetManagerProxy() override;
   std::unique_ptr<org::chromium::flimflam::ServiceProxyInterface>
-  GetServiceForPath(const std::string& path) override;
+  GetServiceForPath(const dbus::ObjectPath& path) override;
 
   // Sets the service_proxy that will be returned by GetServiceForPath().
   void SetServiceForPath(
-      const std::string& path,
+      const dbus::ObjectPath& path,
       std::unique_ptr<org::chromium::flimflam::ServiceProxyInterface>
           service_proxy);