style: Use the style system's LengthPercentage for shape-margin.

This also makes us pass a few WPTs because we stop losing precision when
serializing the computed value.

Differential Revision: https://phabricator.services.mozilla.com/D17738

components/style/properties/gecko.mako.rs

@@ -1396,7 +1396,7 @@ impl Clone for ${style_struct.gecko_struct_name} {
         "MozLength": impl_style_coord,
         "MozScriptMinSize": impl_absolute_length,
         "MozScriptSizeMultiplier": impl_simple,
-        "NonNegativeLengthPercentage": impl_style_coord,
+        "NonNegativeLengthPercentage": impl_simple,
         "NonNegativeNumber": impl_simple,
         "Number": impl_simple,
         "Opacity": impl_simple,

components/style/values/generics/mod.rs

@@ -174,6 +174,7 @@ impl SpecifiedValueInfo for CounterStyleOrNone {
     ToComputedValue,
     ToCss,
 )]
+#[repr(transparent)]
 pub struct NonNegative<T>(pub T);
 
 /// A wrapper of greater-than-or-equal-to-one values.