Add layout RPC query for getting an element's style

This enables us to implement Element::has_css_layout_box() in a more
direct way, and also enables us to remove some of the existing more
specific queries.

Fixes #19811.

components/style/values/specified/box.rs

@@ -216,6 +216,12 @@ impl Display {
             other => other,
         }
     }
+
+    /// Returns true if the value is `None`
+    #[inline]
+    pub fn is_none(&self) -> bool {
+        *self == Display::None
+    }
 }
 
 /// A specified value for the `vertical-align` property.