getDOMNodeID: Don't return .id on random objects

If you defined a global named `id` (a horrible name, I know) then `getDOMNodeID(window)` would return that object. Since only DOM nodes can have IDs, this should be a noop change otherwise.

Test Plan:
Verified that `document.documentElement` and `document.body` both support `getAttributeNode` in latest Chrome and in IE8.