It would appear that shadows were expected to render above underlines,
violating the CSS3 spec for text-shadow
(http://dev.w3.org/csswg/css3-text/#text-shadow). The text-overflow
reftest relied on our previous standards-mode decoration implementation,
which drew strikethroughs after drawing the contents of the box,
regardless of whether there was any text; the reference thus no longer
needs to artificially introduce the decoration.
