mash: Fix MusPropertyMirrorAsh for owned properties; add test.

ash/public/cpp/mus_property_mirror_ash.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_MUS_PROPERTY_MIRROR_H_
+#define ASH_PUBLIC_CPP_MUS_PROPERTY_MIRROR_H_
+
+#include "base/macros.h"
+#include "ui/views/mus/mus_property_mirror.h"
+
+namespace ash {
+
+// Relays aura content window properties to its root window (the mash frame).
+// Ash relies on various window properties for frame titles, shelf items, etc.
+// These properties are read from the client's root, not child content windows.
+class MusPropertyMirrorAsh : public views::MusPropertyMirror {
+ public:
+  MusPropertyMirrorAsh();
+  ~MusPropertyMirrorAsh() override;
+
+  // MusPropertyMirror:
+  void MirrorPropertyFromWidgetWindowToRootWindow(aura::Window* window,
+                                                  aura::Window* root_window,
+                                                  const void* key) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MusPropertyMirrorAsh);
+};
+
+}  // namespace ash
+
+#endif  // ASH_PUBLIC_CPP_MUS_PROPERTY_MIRROR_H_