Chrome browsers with nonstandard zoom report fractional devicePixelRatio.

Fractional pixel values cause drawing artifacts with text.
Tested on a retina display (rounds up to 2) and a standard
display (stays at 1).

Before:
![screen shot 2014-09-12 at 8 38 55 am](https://cloud.githubusercontent.com/assets/15394/4252356/71fd281c-3a95-11e4-9424-6ecbb08d104a.png)

After:
![screen shot 2014-09-12 at 8 37 41 am](https://cloud.githubusercontent.com/assets/15394/4252361/79b1ee58-3a95-11e4-8cc2-3d7468bb1ef5.png)