No need to delete ax subtree when new node is added.

ui/accessibility/ax_tree_serializer.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_ACCESSIBILITY_AX_TREE_SERIALIZER_H_
 #define UI_ACCESSIBILITY_AX_TREE_SERIALIZER_H_
 
 #include <set>
 
 #include "base/containers/hash_tables.h"
@@ skipping 275 matching lines @@
 
 template<typename AXSourceNode>
 void AXTreeSerializer<AXSourceNode>::SerializeChanges(
     AXSourceNode node,
     AXTreeUpdate* out_update) {
   // If the node isn't in the client tree, we need to serialize starting
   // with the LCA.
   AXSourceNode lca = LeastCommonAncestor(node);
 
   if (client_root_) {
-    // If the LCA is anything other than the node itself, tell the
-    // client to first delete the subtree rooted at the LCA.
-    bool need_delete = !tree_->IsEqual(lca, node);
+    bool need_delete = false;
     if (tree_->IsValid(lca)) {
       // Check for any reparenting within this subtree - if there is
       // any, we need to delete and reserialize the whole subtree
       // that contains the old and new parents of the reparented node.
       if (AnyDescendantWasReparented(lca, &lca))
         need_delete = true;
     }
 
     if (!tree_->IsValid(lca)) {
       // If there's no LCA, just tell the client to destroy the whole
@@ skipping 144 matching lines @@
   }
 
   // Serialize all of the new children, recursively.
   for (size_t i = 0; i < children_to_serialize.size(); ++i)
     SerializeChangedNodes(children_to_serialize[i], out_update);
 }
 
 }  // namespace ui
 
 #endif  // UI_ACCESSIBILITY_AX_TREE_SERIALIZER_H_