Investigate fingerprinting display DPI

So far, we've not bothered spoofing DPI because there does not seem to be a large amount of variability. We should perhaps study this (#6119 (moved)?) and see if it's actually true. Here's a simple JS-based test: http://browserspy.dk/resolution.php

This information is also now available to CSS: https://developer.mozilla.org/en-US/docs/CSS/resolution