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.

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components/style/servo/restyle_damage.rs

@@ -60,7 +60,8 @@ 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(old: &ComputedValues,
+    pub fn compute_style_difference(_source: &ComputedValues,
+                                    old: &ComputedValues,
                                     new: &ComputedValues)
                                     -> StyleDifference {
         let damage = compute_damage(old, new);