[cssom-1] Serialization of feature values

[cdoublev]

Serializing Media Feature Values is poorly defined.

This is not really important but hey, it could simply be replaced with serializing a CSS component value: keywords in lowercase, base 10 numbers, etc.

In current version of Chrome and FF:

Input	Chrome	FF	Expected
orientation: PORTRAIT	orientation: portrait	orientation: portrait	orientation: portrait
aspect-ratio: 1	aspect-ratio: 1 / 1	aspect-ratio: 1 / 1	aspect-ratio: 1 / 1
width: 0	width: 0	width: 0px	width: 0px
width: 1.0px	width: 1px	width: 1px	width: 1px
width: calc(1px * 2)	width: calc(2px)	width: calc(2px)	width: calc(2px)