style: Use NonNegative more in the border code.

This ended up not being so small of a patch as I'd have thought, since it
propagated a bit. But most of it is mechanical. Interesting part is
NonNegativeNumberOrPercentage and the actual uses of the NonNegative stuff and
during parsing.

This looks like it'd fix a few correctness issues during interpolation for all
the types except for BorderRadius and co (which handled it manually).

I should write tests for those in a different patch.

Differential Revision: https://phabricator.services.mozilla.com/D14673

components/style/values/computed/mod.rs

@@ -543,6 +543,15 @@ pub enum NumberOrPercentage {
     Number(Number),
 }
 
+impl NumberOrPercentage {
+    fn clamp_to_non_negative(self) -> Self {
+        match self {
+            NumberOrPercentage::Percentage(p) => NumberOrPercentage::Percentage(p.clamp_to_non_negative()),
+            NumberOrPercentage::Number(n) => NumberOrPercentage::Number(n.max(0.)),
+        }
+    }
+}
+
 impl ToComputedValue for specified::NumberOrPercentage {
     type ComputedValue = NumberOrPercentage;
 
@@ -572,6 +581,31 @@ impl ToComputedValue for specified::NumberOrPercentage {
     }
 }
 
+/// A non-negative <number-percentage>.
+pub type NonNegativeNumberOrPercentage = NonNegative<NumberOrPercentage>;
+
+impl NonNegativeNumberOrPercentage {
+    /// Returns the `100%` value.
+    #[inline]
+    pub fn hundred_percent() -> Self {
+        NonNegative(NumberOrPercentage::Percentage(Percentage::hundred()))
+    }
+}
+
+impl ToAnimatedValue for NonNegativeNumberOrPercentage {
+    type AnimatedValue = NumberOrPercentage;
+
+    #[inline]
+    fn to_animated_value(self) -> Self::AnimatedValue {
+        self.0
+    }
+
+    #[inline]
+    fn from_animated_value(animated: Self::AnimatedValue) -> Self {
+        NonNegative(animated.clamp_to_non_negative())
+    }
+}
+
 /// A type used for opacity.
 pub type Opacity = CSSFloat;