Reimplement automation API on top of C++-backed AXTree.

ui/accessibility/ax_tree.cc

Index: ui/accessibility/ax_tree.cc
diff --git a/ui/accessibility/ax_tree.cc b/ui/accessibility/ax_tree.cc
index d07f239b79d9a292b1221239067b71ee2b715e03..8ae61c67d01d00003e63d1859e372f6aec4cda68 100644
--- a/ui/accessibility/ax_tree.cc
+++ b/ui/accessibility/ax_tree.cc
@@ -221,11 +221,11 @@ void AXTree::DestroySubtree(AXNode* node,
 
 void AXTree::DestroyNodeAndSubtree(AXNode* node,
                                    AXTreeUpdateState* update_state) {
+  if (delegate_)
+    delegate_->OnNodeWillBeDeleted(this, node);
   id_map_.erase(node->id());
   for (int i = 0; i < node->child_count(); ++i)
     DestroyNodeAndSubtree(node->ChildAtIndex(i), update_state);
-  if (delegate_)
-    delegate_->OnNodeWillBeDeleted(this, node);
   if (update_state) {
     update_state->pending_nodes.erase(node);
   }