Factors out DOM traversal, keeping the code in parallel free of traversal-specific logic.

DOM traversals and Flow traversals look very similar. This patch unifies them
with the preorder/postorder pattern. Hopefully, it also opens the door for writing
the traversal code only once, instead of the duplication we have today.

components/layout/wrapper.rs

@@ -888,3 +888,15 @@ pub unsafe fn layout_node_from_unsafe_layout_node(node: &UnsafeLayoutNode) -> La
     let (node, _) = *node;
     mem::transmute(node)
 }
+
+/// A top-down traversal.
+pub trait PreorderDOMTraversal {
+    /// The operation to perform. Return true to continue or false to stop.
+    fn process(&self, _node: LayoutNode);
+}
+
+/// A bottom-up traversal, with a optional in-order pass.
+pub trait PostorderDOMTraversal {
+    /// The operation to perform. Return true to continue or false to stop.
+    fn process(&self, _node: LayoutNode);
+}