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/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 {