Auto merge of #18450 - upsuper:property-is-set, r=Manishearth

Use the LonghandIdSet to check whether a property is set

Servo side change of [bug 1398479](https://bugzilla.mozilla.org/show_bug.cgi?id=1398479).

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components/style/properties/declaration_block.rs

@@ -239,6 +239,11 @@ impl PropertyDeclarationBlock {
         !self.declarations_importance.all_true()
     }
 
+    /// Returns whether this block contains a declaration of a given longhand.
+    pub fn contains(&self, id: LonghandId) -> bool {
+        self.longhands.contains(id)
+    }
+
     /// Get a declaration for a given property.
     ///
     /// NOTE: This is linear time.

ports/geckolib/glue.rs

@@ -2566,10 +2566,8 @@ pub extern "C" fn Servo_DeclarationBlock_PropertyIsSet(declarations:
                                                        RawServoDeclarationBlockBorrowed,
                                                        property: nsCSSPropertyID)
         -> bool {
-    use style::properties::PropertyDeclarationId;
-    let long = get_longhand_from_id!(property);
     read_locked_arc(declarations, |decls: &PropertyDeclarationBlock| {
-        decls.get(PropertyDeclarationId::Longhand(long)).is_some()
+        decls.contains(get_longhand_from_id!(property))
     })
 }