[cssom] Color properties should use "used value" as "resolved value"

[upsuper]

In the new CSS Color spec, computed value of <color> could be either a numeric color or currentcolor keyword. However, color properties currently always return numeric color value, and web content may depend on that behavior. I think we should probably make those properties use "used value" as "resolved value" for getComputedStyle.

[upsuper]

Also that matches behavior of browsers which have implemented computed-value time currentcolor keyword.

[frivoal]

Agenda+ because w3c/csswg-test#1161 is blocked by this.

[tabatkins]

WG resolved to accept this.

[svgeesus]

also clarified on call, no impact of this change on css3-color or css4-color.

[frivoal]

The WG resolved (ah!) in favor of making <color> properties use used values as resolved values.

@therealglazou @zcorpan I think you guys are the editors, can you do the update?