Mac a11y: Add Help, TopLevelUIElement, and Window accessibility attributes.

ui/accessibility/platform/test_ax_node_wrapper.cc

Index: ui/accessibility/platform/test_ax_node_wrapper.cc
diff --git a/ui/accessibility/platform/test_ax_node_wrapper.cc b/ui/accessibility/platform/test_ax_node_wrapper.cc
index f2b91731aa4162671f9e49f3748d7eacb69b95df..9e2898d49a5f88d989a6fc77f63dc9f0145e2275 100644
--- a/ui/accessibility/platform/test_ax_node_wrapper.cc
+++ b/ui/accessibility/platform/test_ax_node_wrapper.cc
@@ -71,6 +71,10 @@ const AXNodeData& TestAXNodeWrapper::GetData() {
   return node_->data();
 }
 
+gfx::NativeWindow TestAXNodeWrapper::GetTopLevelWidget() {
+  return nullptr;
+}
+
 gfx::NativeViewAccessible TestAXNodeWrapper::GetParent() {
   TestAXNodeWrapper* parent_wrapper = GetOrCreate(tree_, node_->parent());
   return parent_wrapper ?