Add a window property to associate RWHVA with its child AX tree ID

ui/accessibility/platform/aura_window_properties.h

Index: ui/accessibility/platform/aura_window_properties.h
diff --git a/ui/accessibility/platform/aura_window_properties.h b/ui/accessibility/platform/aura_window_properties.h
new file mode 100644
index 0000000000000000000000000000000000000000..8c8ea138c3da9c486cee51a83a7ce19d6146108e
--- /dev/null
+++ b/ui/accessibility/platform/aura_window_properties.h
@@ -0,0 +1,19 @@
+// 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.
+
+#ifndef UI_ACCESSIBILITY_PLATFORM_AURA_WINDOW_PROPERTIES_H_
+#define UI_ACCESSIBILITY_PLATFORM_AURA_WINDOW_PROPERTIES_H_
+
+#include "ui/accessibility/ax_export.h"
+#include "ui/accessibility/ax_tree_id_registry.h"
+#include "ui/aura/window.h"
+
+namespace ui {
+
+AX_EXPORT extern const aura::WindowProperty<AXTreeIDRegistry::AXTreeID>* const
+    kChildAXTreeID;
+
+}  // namespace ui
+
+#endif  // UI_ACCESSIBILITY_PLATFORM_AURA_WINDOW_PROPERTIES_H_