Fix nits in GCedTreeNode<>.

- Remove reference to RetainPtr<> from comments.
- Make GarbageCollected<> leftmost base class per garbage-collected.h

Change-Id: Ie2217116eaaff48e9264ca54399b600859bd6cfc
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/71750
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

fxjs/gc/gced_tree_node.h

diff --git a/fxjs/gc/gced_tree_node.h b/fxjs/gc/gced_tree_node.h
index a983fe5..337f73a 100644
--- a/fxjs/gc/gced_tree_node.h
+++ b/fxjs/gc/gced_tree_node.h
@@ -13,12 +13,10 @@
 
 namespace fxjs {
 
-// For DOM/XML-ish trees, where references outside the tree are RetainPtr<T>,
-// and the parent node also "retains" its children but doesn't always have
-// a direct pointer to them.
+// For DOM/XML-ish trees, where references outside the tree are Persistent<>.
 template <typename T>
-class GCedTreeNode : public TreeNode<T, cppgc::Member<T>>,
-                     public cppgc::GarbageCollected<GCedTreeNode<T>> {
+class GCedTreeNode : public cppgc::GarbageCollected<GCedTreeNode<T>>,
+                     public TreeNode<T, cppgc::Member<T>> {
  public:
   using TreeNode<T, cppgc::Member<T>>::RemoveChild;