IDL: Fix handling of nullable NodeFilter / XPathNSResolver arguments

Arguments of NodeFilter and XPathNSResolver have a special case that adds
a ".release()" suffix to the argument expression. This special case didn't
trigger if the argument was nullable.

Review URL: https://codereview.chromium.org/340783012

git-svn-id: svn://svn.chromium.org/blink/trunk@176933 bbb929c8-8fbe-4397-9dbb-9b2b20218538