style: Invalidate style for ::selection.

This patch invalidates the style for `::selection`, which will restore the
behavior before the regression.

However, it's still not quite correct, because repaint is not triggered. Given
that `::selection` requires some major change to implement
https://github.com/w3c/csswg-drafts/issues/2474, we can address this problem
later.

Differential Revision: https://phabricator.services.mozilla.com/D35305

components/style/gecko/pseudo_element.rs

@@ -116,6 +116,12 @@ impl PseudoElement {
         *self == PseudoElement::Marker
     }
 
+    /// Whether this pseudo-element is the ::selection pseudo.
+    #[inline]
+    pub fn is_selection(&self) -> bool {
+        *self == PseudoElement::Selection
+    }
+
     /// Whether this pseudo-element is ::first-letter.
     #[inline]
     pub fn is_first_letter(&self) -> bool {