Auto merge of #17353 - upsuper:counter-style-disc, r=dbaron
Auto merge of #17353 - upsuper:counter-style-disc, r=dbaron

Make counter style 'disc' not overridable

This is the Servo side change for [bug 1372488](https://bugzilla.mozilla.org/show_bug.cgi?id=1372488).

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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17353)
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