Bug 1374233 - Part 5: Use NonNegativeLength and NonNegativeAu for border-spacing.

We already have NonNegativeLength and NonNegativeAu, so we can re-use it
to define the specified value and the computed value of border-spacing.
And then implement ToAnimatedValue for it.

MozReview-Commit-ID: CLckpKMYVXU

components/script/dom/element.rs

@@ -548,7 +548,7 @@ impl LayoutElementHelpers for LayoutJS<Element> {
                 shared_lock,
                 PropertyDeclaration::BorderSpacing(
                     Box::new(border_spacing::SpecifiedValue {
-                        horizontal: width_value,
+                        horizontal: width_value.into(),
                         vertical: None,
                     }))));
         }