style: Forbid negative CSS resolutions at parse time

Apply some clang-tidy suggestions while I was going through the
ServoStyleConstsInlines while at it.

Remove one 0x test because it's tested on the same test and causes a
harness error.

Differential Revision: https://phabricator.services.mozilla.com/D180331

components/style/values/computed/resolution.rs

@@ -36,7 +36,7 @@ impl ToComputedValue for specified::Resolution {
 
     #[inline]
     fn to_computed_value(&self, _: &Context) -> Self::ComputedValue {
-        Resolution(self.dppx())
+        Resolution(crate::values::normalize(self.dppx().max(0.0)))
     }
 
     #[inline]

components/style/values/specified/resolution.rs

@@ -129,7 +129,7 @@ impl Parse for Resolution {
         match *input.next()? {
             Token::Dimension {
                 value, ref unit, ..
-            } => Self::parse_dimension(value, unit)
+            } if value >= 0. => Self::parse_dimension(value, unit)
                 .map_err(|()| location.new_custom_error(StyleParseErrorKind::UnspecifiedError)),
             Token::Function(ref name) => {
                 let function = CalcNode::math_function(context, name, location)?;