[css-values-4] Canonical <resolution> unit

[tabatkins]

Due to WebKit shipping some things using x as a <resolution> unit, we added it to <resolution> officially in Values 4. When adding it, I chose to set x as the canonical unit for the type, as WebKit's additions seemed to be the main use for the type, and it seemed like preserving them as serializing with x was important.

Chrome is planning on finally actually implementing this - accepting x in the MQs that take <resolution>, and accepting other <resolution> units in an (unprefixed) image-set() and <img srcset>. The question that arises is thus whether x is really the best canonical unit to use or not, considering that other browsers don't yet implement x, or whether we should change it to dppx.

[tabatkins]

Whoops, this is a duplicate of #2241.