[cssom] Make resolved value of border-*-width be the used value

[upsuper]

It seems at least Blink and Gecko both agree that the resolved value of border-*-width should be using the used value like padding-* and margin-*. (I supposed Edge does so as well, but my test page crashes Edge so I haven't tried detailedly.)

[upsuper]

border-*-width doesn't accept percentage, so it doesn't actually have a different used value.