[css-typed-om] CSSTokenStream values need to computed to other values in some cases

[tabatkins]

Spec currently says that compute to another CSSTokenStream, just with all the variables resolved. This is true for custom properties, but when any other property is a CSSTokenStream, the computed value is whatever types that property normally has.

[shans]

It's not even true for custom properties that have a registered type.

[shans]

I think the spec actually doesn't say this at all. I rewrote the section to be a lot clearer though.

[shans]

@tabatkins ptal at attached pull request

[shans]

Closed via #296