style: Don't incorrectly clamp values in calc that might not be only lengths.

Expressions with percentages may be negative or positive at computed value time.

So, we can only clamp lengths at computed value time, which is what the other
browsers do.

components/style_traits/values.rs

@@ -63,6 +63,8 @@ macro_rules! __define_css_keyword_enum__actual {
 
 
 pub mod specified {
+    use app_units::Au;
+
     #[repr(u8)]
     #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
     #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -79,5 +81,14 @@ pub mod specified {
                 AllowedNumericType::NonNegative => value >= 0.,
             }
         }
+
+        #[inline]
+        pub fn clamp(&self, val: Au) -> Au {
+            use std::cmp;
+            match *self {
+                AllowedNumericType::All => val,
+                AllowedNumericType::NonNegative => cmp::max(Au(0), val),
+            }
+        }
     }
 }