style: Implement GeckoElement::each_part.

This should make all the pieces come together.

Note that we don't need to look at the snapshot for ::part() for now (other than
when selector-matching normally) because I decided to just restyle the element
for now when the part attribute changes.

::part() can't affect descendants anyway (as long as we don't do the forwarding
stuff), and eager pseudo-elements are handled during the normal element restyle,
so it seems to me that adding all the complexity that we have for classes to
part may not be worth it at least yet.

Differential Revision: https://phabricator.services.mozilla.com/D32648

components/style/gecko/snapshot.rs

@@ -211,7 +211,7 @@ impl ElementSnapshot for GeckoElementSnapshot {
             return;
         }
 
-        snapshot_helpers::each_class(&self.mClass, callback)
+        snapshot_helpers::each_class_or_part(&self.mClass, callback)
     }
 
     #[inline]

components/style/gecko/snapshot_helpers.rs

@@ -107,9 +107,9 @@ pub fn has_class_or_part(
 }
 
 /// Given an item, a callback, and a getter, execute `callback` for each class
-/// this `item` has.
+/// or part name this `item` has.
 #[inline(always)]
-pub fn each_class<F>(attr: &structs::nsAttrValue, mut callback: F)
+pub fn each_class_or_part<F>(attr: &structs::nsAttrValue, mut callback: F)
 where
     F: FnMut(&Atom),
 {

components/style/gecko/wrapper.rs

@@ -1372,7 +1372,19 @@ impl<'le> TElement for GeckoElement<'le> {
             None => return,
         };
 
-        snapshot_helpers::each_class(attr, callback)
+        snapshot_helpers::each_class_or_part(attr, callback)
+    }
+
+    fn each_part<F>(&self, callback: F)
+    where
+        F: FnMut(&Atom),
+    {
+        let attr = match self.get_part_attr() {
+            Some(c) => c,
+            None => return,
+        };
+
+        snapshot_helpers::each_class_or_part(attr, callback)
     }
 
     #[inline]