Auto merge of #12757 - emilio:stylo, r=bholley,pcwalton

stylo: Stop restyling display: none elements, remove the has_changed hack that made us use ReconstructFrame unconditionally.

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

r? @bholley

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12757)
<!-- Reviewable:end -->

components/script_layout_interface/restyle_damage.rs

@@ -47,7 +47,11 @@ impl TRestyleDamage for RestyleDamage {
     /// For Servo the style source is always the computed values.
     type PreExistingComputedValues = Arc<ServoComputedValues>;
 
-    fn compute(old: Option<&Arc<ServoComputedValues>>,
+    fn empty() -> Self {
+        RestyleDamage::empty()
+    }
+
+    fn compute(old: &Arc<ServoComputedValues>,
                new: &Arc<ServoComputedValues>) -> RestyleDamage {
         compute_damage(old, new)
     }
@@ -150,13 +154,7 @@ macro_rules! add_if_not_equal(
     })
 );
 
-fn compute_damage(old: Option<&Arc<ServoComputedValues>>, new: &Arc<ServoComputedValues>) -> RestyleDamage {
-    let new = &**new;
-    let old: &ServoComputedValues = match old {
-        None => return RestyleDamage::rebuild_and_reflow(),
-        Some(cv) => &**cv,
-    };
-
+fn compute_damage(old: &ServoComputedValues, new: &ServoComputedValues) -> RestyleDamage {
     let mut damage = RestyleDamage::empty();
 
     // This should check every CSS property, as enumerated in the fields of