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`
It is to ensure the same vertical browser scrollbars for this element and content element. 
We have the case differents behavior ` overflow: scroll; ` and ` overflow-x: hidden; overflow-y: scroll; ` at OS X 10.9.4.  In addition, probably, we have to divide sizes of the vertical and horizontal browser scrollbars.