Auto merge of #17796 - heycam:calc-difference, r=emilio

style: pass old ComputedValues from Element data into Gecko_CalcStyleDifference

This is the Servo-side part of https://bugzilla.mozilla.org/show_bug.cgi?id=1380133.

<!-- 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/17796)
<!-- Reviewable:end -->

components/style/properties/gecko.mako.rs

@@ -137,6 +137,10 @@ impl ComputedValues {
         let atom = Atom::from(atom);
         PseudoElement::from_atom(&atom)
     }
+
+    pub fn as_style_context(&self) -> &::gecko_bindings::structs::mozilla::ServoStyleContext {
+        &self.0
+    }
 }
 
 impl Drop for ComputedValues {