Add null check when reporting changed nodes to an AX tree.

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 43e635af1a207bede9fe8d71f0e0ef1e3e5da326..ed5c6f744e3e8120ca3b4b4cd0fa9ea23f627362 100644
--- a/ui/accessibility/ax_tree.cc
+++ b/ui/accessibility/ax_tree.cc
@@ -279,6 +279,9 @@ bool AXTree::Unserialize(const AXTreeUpdate& update) {
     changes.reserve(update.nodes.size());
     for (size_t i = 0; i < update.nodes.size(); ++i) {
       AXNode* node = GetFromId(update.nodes[i].id);
+      if (!node)
+        continue;
+
       bool is_new_node = new_nodes.find(node) != new_nodes.end();
       bool is_reparented_node =
           is_new_node && update_state.HasRemovedNode(node);