Bug 1374233 - Part 2: Add NonNegativeAu.

Add values::computed::NonNegativeAu, so BorderSideWith could be computed
to this non-negative Au, for the following properties:
1. outline-width
2. border-{*}-width
3. column-rule-width
4. -webkit-text-stroke-width

MozReview-Commit-ID: ASHaB2F7VtM

components/layout/display_list_builder.rs

@@ -1572,7 +1572,7 @@ impl FragmentDisplayListBuilding for Fragment {
                                                     clip: &Rect<Au>) {
         use style::values::Either;
 
-        let width = style.get_outline().outline_width;
+        let width = style.get_outline().outline_width.0;
         if width == Au(0) {
             return
         }