style: Introduce a generic way of gathering information from the cascade, and use it for viewport units.

components/style/custom_properties.rs

@@ -39,6 +39,12 @@ pub struct SpecifiedValue {
     references: HashSet<Name>,
 }
 
+impl ::values::HasViewportPercentage for SpecifiedValue {
+    fn has_viewport_percentage(&self) -> bool {
+        panic!("has_viewport_percentage called before resolving!");
+    }
+}
+
 pub struct BorrowedSpecifiedValue<'a> {
     css: &'a str,
     first_token_type: TokenSerializationType,