fix browser scrollbar width detection

System: Mac,
Browser: Chrome 37.

Overflow of nano-content:
```css
overflow-x: hidden;
overflow-y: scroll;
```
Real scroll width 16px, but `BROWSER_SCROLLBAR_WIDTH === 0`
Yes. We have a custom styles, and did not see https://github.com/jamesflorentino/nanoScrollerJS/issues/255.