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.cc b/shill_proxy.cc
index 99817ea..1c050b4 100644
--- a/shill_proxy.cc
+++ b/shill_proxy.cc
@@ -35,10 +35,9 @@
 }
 
 std::unique_ptr<ServiceProxyInterface> ShillProxy::GetServiceForPath(
-    const std::string& path) {
+    const dbus::ObjectPath& path) {
   DCHECK(bus_.get());
-  return std::unique_ptr<ServiceProxyInterface>(
-      new ServiceProxy(bus_, dbus::ObjectPath(path)));
+  return std::unique_ptr<ServiceProxyInterface>(new ServiceProxy(bus_, path));
 }
 
 }  // namespace chromeos_update_engine