style: Propagate changes in custom properties' computed values to descendants.

If ComputedValues.custom_properties differs between the old and new
ComputedValues, indicate that we have to propogate changes to
descendants by setting child_cascade_requirement to
MustCascadeDescendants in cascade_primary.

style::matching::TElement::cascade_primary already calls
accumulate_damage, which eventually calls
ServoRestyleDamage::compute_style_difference in order to check if other
properties' computed values changed. If any of those change, we signal
that we need to propogate changes for inherited properties.

With Properties & Values, some custom properties will not be inherited,
and we will need to revisit this.

components/style/properties/properties.mako.rs

@@ -1894,6 +1894,13 @@ impl ComputedValues {
         self.visited_style.clone()
     }
 
+    // Aah! The << in the return type below is not valid syntax, but we must
+    // escape < that way for Mako.
+    /// Gets a reference to the custom properties map (if one exists).
+    pub fn get_custom_properties(&self) -> Option<<&::custom_properties::ComputedValuesMap> {
+        self.custom_properties.as_ref().map(|x| &**x)
+    }
+
     /// Get the custom properties map if necessary.
     ///
     /// Cloning the Arc here is fine because it only happens in the case where