servo: Merge #18450 - Use the LonghandIdSet to check whether a property is set (from upsuper:property-is-set); r=Manishearth

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

Source-Repo: https://github.com/servo/servo
Source-Revision: 24b2d8d9cf265a149535ebc22941e427dd5e21b3

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 5a2be5b5ff4886cce99883d4338bad2d6621527e

servo/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.

servo/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))
     })
 }