[css-lists] size of images used as list-style-image

The css-lists spec does not (as far as I can see) say anything about the
size of images used for the list-style-image property. It probably should,
but I don't have a proposal for what.

Currently Firefox and Chrome use the size of the image referred to by the
url if a url is used. Chrome looks like it uses 1em by 1em if a gradient is
specified. I am currently implementing non-url images in Firefox, I'll
probably use 1em by 1em for now unless anyone has a better suggestion.

Thanks, Nick.