[css-typed-om] CSSURLImageValue url setter should be specified

[dbaron]

The setter and getter for the url attribute of CSSURLImageValue should be specified. The description of the setter should address the same issues as #630.

[darrnshn]

CSSURLImageValue.url is readonly, so I think we only need to add a description for the getter?

[dbaron]

Er, I guess url is currently readonly... but why should it be readonly?

[dbaron]

(Being readonly seems inconsistent with the rest of the typed OM value classes.)

[darrnshn]

Sorry, I'm not really sure :/ Paging @tabatkins .

[tabatkins]

Unsure too, this part was written by Shane and I haven't given it a serious look since. I'll check this out.