stylo: Remove a lot of the restyle damage related complexity.

The only reason why we had the `existing_style_for_style_damage` bit is to apply
some optimizations that we don't have anymore.

I still want to reintroduce a few of them, at least for the non-eager
pseudo-element case... But I think I won't need this at all.

This allows us to remove a fair amount of Gecko code too.

components/style/servo/restyle_damage.rs

@@ -60,26 +60,15 @@ impl HeapSizeOf for ServoRestyleDamage {
 impl ServoRestyleDamage {
     /// Compute the `StyleDifference` (including the appropriate restyle damage)
     /// for a given style change between `old` and `new`.
-    pub fn compute_style_difference(_source: &ComputedValues,
-                                    old: &ComputedValues,
-                                    new: &ComputedValues)
-                                    -> StyleDifference {
+    pub fn compute_style_difference(
+        old: &ComputedValues,
+        new: &ComputedValues,
+    ) -> StyleDifference {
         let damage = compute_damage(old, new);
         let change = if damage.is_empty() { StyleChange::Unchanged } else { StyleChange::Changed };
         StyleDifference::new(damage, change)
     }
 
-    /// Computes the `StyleDifference` between the two `ComputedValues` objects
-    /// for the case where the old and new style are both `display: none`.
-    ///
-    /// For Servo we never need to generate any damage for such elements.
-    pub fn compute_undisplayed_style_difference(
-        _old_style: &ComputedValues,
-        _new_style: &ComputedValues,
-    ) -> StyleDifference {
-        StyleDifference::new(Self::empty(), StyleChange::Unchanged)
-    }
-
     /// Returns a bitmask that represents a flow that needs to be rebuilt and
     /// reflowed.
     ///

components/style/servo/selector_parser.rs

@@ -130,10 +130,22 @@ impl PseudoElement {
         result
     }
 
-    /// Whether the current pseudo element is :before or :after.
+    /// Whether the current pseudo element is ::before or ::after.
     #[inline]
     pub fn is_before_or_after(&self) -> bool {
-        matches!(*self, PseudoElement::After | PseudoElement::Before)
+        self.is_before() || self.is_after()
+    }
+
+    /// Whether this pseudo-element is the ::before pseudo.
+    #[inline]
+    pub fn is_before(&self) -> bool {
+        *self == PseudoElement::Before
+    }
+
+    /// Whether this pseudo-element is the ::after pseudo.
+    #[inline]
+    pub fn is_after(&self) -> bool {
+        *self == PseudoElement::After
     }
 
     /// Whether the current pseudo element is :first-letter