Re-land: Add a window property to associate RWHVA with its child AX tree ID

ui/accessibility/platform/aura_window_properties.cc

Index: ui/accessibility/platform/aura_window_properties.cc
diff --git a/ui/accessibility/platform/aura_window_properties.cc b/ui/accessibility/platform/aura_window_properties.cc
new file mode 100644
index 0000000000000000000000000000000000000000..146ad75b77e8ec6738db2abb1631ebac956776d0
--- /dev/null
+++ b/ui/accessibility/platform/aura_window_properties.cc
@@ -0,0 +1,15 @@
+// Copyright 2017 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.
+
+#include "ui/accessibility/platform/aura_window_properties.h"
+
+#include "ui/base/class_property.h"
+
+namespace ui {
+
+DEFINE_UI_CLASS_PROPERTY_KEY(AXTreeIDRegistry::AXTreeID,
+                             kChildAXTreeID,
+                             AXTreeIDRegistry::kNoAXTreeID);
+
+}  // namespace ui