style: Share custom property declarations if they don't contain variable references.

components/style/values/generics/image.rs

@@ -9,6 +9,7 @@
 use Atom;
 use cssparser::serialize_identifier;
 use custom_properties;
+use servo_arc::Arc;
 use std::fmt;
 use style_traits::ToCss;
 
@@ -153,7 +154,8 @@ pub struct PaintWorklet {
     pub name: Atom,
     /// The arguments for the worklet.
     /// TODO: store a parsed representation of the arguments.
-    pub arguments: Vec<custom_properties::SpecifiedValue>,
+    #[cfg_attr(feature = "servo", ignore_heap_size_of = "Arc")]
+    pub arguments: Vec<Arc<custom_properties::SpecifiedValue>>,
 }
 
 trivial_to_computed_value!(PaintWorklet);

components/style/values/specified/image.rs

@@ -917,7 +917,7 @@ impl Parse for PaintWorklet {
             let name = Atom::from(&**input.expect_ident()?);
             let arguments = input.try(|input| {
                 input.expect_comma()?;
-                input.parse_comma_separated(|input| Ok(*SpecifiedValue::parse(input)?))
+                input.parse_comma_separated(|input| SpecifiedValue::parse(input))
             }).unwrap_or(vec![]);
             Ok(PaintWorklet { name, arguments })
         })