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/style/values/animated/mod.rs

@@ -9,8 +9,10 @@
 //! module's raison d'être is to ultimately contain all these types.
 
 use app_units::Au;
+use std::cmp::max;
 use values::computed::Angle as ComputedAngle;
 use values::computed::GreaterThanOrEqualToOneNumber as ComputedGreaterThanOrEqualToOneNumber;
+use values::computed::NonNegativeAu;
 use values::computed::NonNegativeNumber as ComputedNonNegativeNumber;
 use values::specified::url::SpecifiedUrl;
 
@@ -118,6 +120,20 @@ impl ToAnimatedValue for ComputedGreaterThanOrEqualToOneNumber {
     }
 }
 
+impl ToAnimatedValue for NonNegativeAu {
+    type AnimatedValue = Self;
+
+    #[inline]
+    fn to_animated_value(self) -> Self {
+        self
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        max(animated.0, Au(0)).into()
+    }
+}
+
 /// Returns a value similar to `self` that represents zero.
 pub trait ToAnimatedZero: Sized {
     /// Returns a value that, when added with an underlying value, will produce the underlying