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/shill_proxy_interface.h b/shill_proxy_interface.h
index 0092f12..5f6b44e 100644
--- a/shill_proxy_interface.h
+++ b/shill_proxy_interface.h
@@ -21,6 +21,7 @@
 #include <string>
 
 #include <base/macros.h>
+#include <dbus/object_path.h>
 #include <shill/dbus-proxies.h>
 
 namespace chromeos_update_engine {
@@ -41,7 +42,7 @@
   // Return a ServiceProxy for the given path. The ownership of the returned
   // instance is transferred to the caller.
   virtual std::unique_ptr<org::chromium::flimflam::ServiceProxyInterface>
-  GetServiceForPath(const std::string& path) = 0;
+  GetServiceForPath(const dbus::ObjectPath& path) = 0;
 
  protected:
   ShillProxyInterface() = default;