style: restrict CSS properties for '::cue'.

According to the spec [1], only those CSS properties listed on the spec can be applied on the `::cue`.

[1] https://www.w3.org/TR/webvtt1/#the-cue-pseudo-element

Differential Revision: https://phabricator.services.mozilla.com/D34262

components/style/gecko/pseudo_element.rs

@@ -186,6 +186,7 @@ impl PseudoElement {
             PseudoElement::FirstLetter => Some(PropertyFlags::APPLIES_TO_FIRST_LETTER),
             PseudoElement::FirstLine => Some(PropertyFlags::APPLIES_TO_FIRST_LINE),
             PseudoElement::Placeholder => Some(PropertyFlags::APPLIES_TO_PLACEHOLDER),
+            PseudoElement::Cue => Some(PropertyFlags::APPLIES_TO_CUE),
             _ => None,
         }
     }