Bug 1505489 - Don't go through all the part names if not there. r=jwatt

This uses the bit added for tracking part attributes in order to avoid doing
wasted work.

Depends on D32643

Differential Revision: https://phabricator.services.mozilla.com/D32644

--HG--
extra : moz-landing-system : lando

servo/components/style/gecko/wrapper.rs

@@ -590,6 +590,9 @@ impl<'le> GeckoElement<'le> {
 
     #[inline(always)]
     fn get_part_attr(&self) -> Option<&structs::nsAttrValue> {
+        if !self.has_part_attr() {
+            return None;
+        }
         snapshot_helpers::find_attr(self.attrs(), &atom!("part"))
     }
 
@@ -623,6 +626,11 @@ impl<'le> GeckoElement<'le> {
         }
     }
 
+    #[inline]
+    fn has_part_attr(&self) -> bool {
+        self.as_node().get_bool_flag(nsINode_BooleanFlag::ElementHasPart)
+    }
+
     #[inline]
     fn may_have_anonymous_children(&self) -> bool {
         self.as_node()