use `win.` in `.viewport`

We had an issue on some 3rd party site due to `self` being globally rewritten by other script
While that's obviously not Bonzo's fail it's quite possible to accure due to non-experienced programmers copying `self = this` pattern and forgetting `var`.

I think the change is also healthy due to making the code more consistent — that was actually the only place referencing to `self`.