Move children_to_process to layout.

We don't need this for Gecko, and it's hard to implement in that case because
there's nowhere obvious to put it (we don't plan to create TSDs for non-dirty
nodes, and non-dirty nodes can have dirty children which require the
children_to_process atomic). There are various solutions here, but punting is
the easiest.

We'll need to rethink this if/when we need to do a bottom-up traversal for
Gecko.

components/script/layout_wrapper.rs

@@ -54,6 +54,7 @@ use std::fmt;
 use std::marker::PhantomData;
 use std::mem::transmute;
 use std::sync::Arc;
+use std::sync::atomic::Ordering;
 use string_cache::{Atom, Namespace};
 use style::atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
 use style::attr::AttrValue;
@@ -221,6 +222,18 @@ impl<'ln> TNode for ServoLayoutNode<'ln> {
         self.node.set_flag(CAN_BE_FRAGMENTED, value)
     }
 
+    fn store_children_to_process(&self, n: isize) {
+        let data = self.get_partial_layout_data().unwrap().borrow();
+        data.parallel.children_to_process.store(n, Ordering::Relaxed);
+    }
+
+    fn did_process_child(&self) -> isize {
+        let data = self.get_partial_layout_data().unwrap().borrow();
+        let old_value = data.parallel.children_to_process.fetch_sub(1, Ordering::Relaxed);
+        debug_assert!(old_value >= 1);
+        old_value - 1
+    }
+
     fn borrow_data(&self) -> Option<AtomicRef<PersistentStyleData>> {
         self.get_style_data().map(|d| d.borrow())
     }