Auto merge of #6251 - glennw:fix-overflow, r=nox

This fixes layers being created with a 3x3 overflow that's not needed.

This exposes https://github.com/servo/servo/issues/6250, so update the affected reftest to use ahem font until it is fixed.

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6251)
<!-- Reviewable:end -->

components/style/values.rs

@@ -919,7 +919,8 @@ pub mod computed {
         pub border_bottom_present: bool,
         pub border_left_present: bool,
         pub is_root_element: bool,
-        pub viewport_size: Size2D<Au>
+        pub viewport_size: Size2D<Au>,
+        pub outline_style_present: bool,
         // TODO, as needed: viewport size, etc.
     }