Only count outline width in overflow calculation if outline style is active.

This fixes layers being created with a 3x3 border 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.

components/style/values.rs

@@ -920,7 +920,8 @@ 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.
     }