Revert "Backed out changeset b10e6ba9cbdb because gecko part had to be backed out."

This reverts commit b96d96d448.

The fix on the Gecko side is trivial, and I can land it after this lands.

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);