[css-typed-om] CSSURLImageValue constructor probably shouldn't require "url()" around the URL

[dbaron]

As specified today, the [CSSURLImageValue constructor] seems to require that url() be around the URL in the string passed to it, since it depends on the <url> production.

I suspect it shouldn't require url()... but if that was the intent, an example should be given.

The spec should also specify more clearly how the parameter is processed, for example, whether CSS character escapes are processed.

[tabatkins]

I went ahead and matched our resolutions for other constructors, and just took the raw value passed in. No parsing goes on at all.