Reland: mash: Fix MusPropertyMirrorAsh for owned properties; add test.

ash/public/cpp/window_properties.h

+// 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 ASH_PUBLIC_CPP_WINDOW_PROPERTIES_H_
+#define ASH_PUBLIC_CPP_WINDOW_PROPERTIES_H_
+
+#include <stdint.h>
+
+#include "ash/public/cpp/ash_public_export.h"
+#include "ash/public/cpp/shelf_types.h"
+#include "ui/base/class_property.h"
+
+namespace aura {
+template <typename T>
+using WindowProperty = ui::ClassProperty<T>;
+}
+
+namespace ash {
+
+// Shell-specific window property keys for use by ash and its clients.
+
+// Alphabetical sort.
+
+// If true (and the window is a panel), it's attached to its shelf item.
+ASH_PUBLIC_EXPORT extern const aura::WindowProperty<bool>* const
+    kPanelAttachedKey;
+
+// A property key to store the type of a window's shelf item.
+ASH_PUBLIC_EXPORT extern const aura::WindowProperty<int32_t>* const
+    kShelfItemTypeKey;
+
+// Alphabetical sort.
+
+}  // namespace ash
+
+#endif  // ASH_PUBLIC_CPP_WINDOW_PROPERTIES_H_