Define whether fieldset's legend should mask border-image

[zcorpan]

See https://bugzilla.mozilla.org/show_bug.cgi?id=1279185

WebKit/Chromium doesn't mask the border-image. Gecko does (at the legend's border edge). EdgeHTML doesn't render border-image at all on fieldset.

cc @MatsPalmgren

[MatsPalmgren]

I don't see a reason for border-image borders to behave differently from other type of borders. But yes, we should specify somewhere how fieldset borders should be rendered in general.
I have a attached a few more testcases on the bug you referenced above where Firefox renders it differently than Chrome. It appears that Chrome clips the border entirely in the block-axis whereas Firefox only clips it to the extent of the legend's border box (i.e. if the border is thicker than the legend's block-size then the border renders above/below it.)
It actually looks like both UAs handles its image borders and ordinary borders consistently.

[zcorpan]

Indeed there is a difference between browsers with normal borders:

http://software.hixie.ch/utilities/js/live-dom-viewer/saved/6115

WebKit/Chromium/EdgeHTML have the same behavior, so I think I'll specify that.