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`
Alright, got it.

Is there something not working in the way nanoScroller scrollbar currently works? I mean that even if scrollbar width is `0` in Mac OS X Chrome, it is actually taken care of by nanoScroller and the scrollbar still renders correctly.

A Chrome related scrollbar bug was recently fixed in nanoScroller: https://github.com/jamesflorentino/nanoScrollerJS/issues/255