Bug 865260 - Use IsXrayWrapper rather than ObjectIsNativeWrapper in nsWindowSH. r=bz

There are some other uses of ObjectIsNativeWrapper in other scriptable helpers
that are tempting to remove as well, but it's probably just better to wait for
that stuff to just go away. Given that the issue we're running into here is
Window-specific, there's not a pressing need to fix the other stuff.