Added test for whether window dimensions in js are adjusted to be device independent pixels

Relating to this issue https://github.com/Modernizr/Modernizr/issues/1138.

It's roughly a translation of http://jsbin.com/IzEYuCI/3/edit using Modernizr's API. I haven't tested it in any devices yet but will test in a few tomorrow... although I don't have access to many so I guess would need testing on a wider suite of devices at some point before being accepted