mash: Relay some Chrome aura window properties to Ash.

ui/views/mus/mus_property_mirror.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 UI_VIEWS_MUS_MUS_PROPERTY_MIRROR_H_
+#define UI_VIEWS_MUS_MUS_PROPERTY_MIRROR_H_
+
+#include "ui/views/mus/mus_export.h"
+
+namespace aura {
+class Window;
+}
+
+namespace views {
+
+// Facilitates copying mus client window properties to their mash frame windows.
+class VIEWS_MUS_EXPORT MusPropertyMirror {
+ public:
+  virtual ~MusPropertyMirror() {}
+
+  // Called when a property with the given |key| has changed for |window|.
+  // |window| is what mus clients get when calling |widget->GetNativeWindow()|.
+  // |root_window| is the frame that mash constructs in order to render the

[sky, 2017/01/27 23:46:22]

The description of root_window is mildly confusing. Maybe something like:

|root_window| is the top-level window representing the widget that is owned by
the window manager and that the window manager observes changes on.

Is that any better?

[msw, 2017/01/28 00:09:16]

Done.

+  // non-client frame decorations (see ash::mus::NonClientFrameController).
+  // Various ash features rely on property values of mus clients' root windows.
+  virtual void MirrorPropertyFromWidgetWindowToRootWindow(
+      aura::Window* window,
+      aura::Window* root_window,
+      const void* key) = 0;
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_MUS_MUS_PROPERTY_MIRROR_H_