Fix a zooming bug with EM MQs
EM MQs were using calculated font size (which include the zooming factor) in order to translate the expected MQ size to CSS pixels.
At the same time, adjustForAbsoluteZoom was taking the zoom into account to decrease the number of CSS pixels that fit into the viewport.
That created a situation where the zoom is taken into account twice, causing a difference between EM MQs and PX MQs.
That wasn't as bad as it could be because the calculated font size wasn't updating itself on every zoom step, but there was still visible difference.
This change simply turns off font calculation for the CSS pixel computation, which results in parity between PX and EM MQs.
I also added a layout test that covers this issue.