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/custom_properties.rs

@@ -66,7 +66,7 @@ pub struct BorrowedSpecifiedValue<'a> {
 
 /// A computed value is just a set of tokens as well, until we resolve variables
 /// properly.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Eq, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct ComputedValue {
     css: String,