Replace uses of for foo in bar.iter(),

and `for foo in bar.iter_mut(), and for foo in bar.into_iter()
(continuation of #7197)

components/layout/flow.rs

@@ -727,7 +727,7 @@ impl AbsoluteDescendants {
     ///
     /// Ignore any static y offsets, because they are None before layout.
     pub fn push_descendants(&mut self, given_descendants: AbsoluteDescendants) {
-        for elem in given_descendants.descendant_links.into_iter() {
+        for elem in given_descendants.descendant_links {
             self.descendant_links.push(elem);
         }
     }