Converts ash to use aura-mus

ash/mus/screen_mus.h

+// Copyright 2016 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_MUS_SCREEN_MUS_H_
+#define ASH_MUS_SCREEN_MUS_H_
+
+#include "base/macros.h"
+#include "ui/display/screen_base.h"
+
+namespace ash {
+
+// Implementation of Screen for mus.
+class ScreenMus : public display::ScreenBase {
+ public:
+  ScreenMus();
+  ~ScreenMus() override;
+
+ private:
+  // display::ScreenBase:
+  display::Display GetDisplayNearestWindow(aura::Window* window) const override;
+
+  DISALLOW_COPY_AND_ASSIGN(ScreenMus);
+};
+
+}  // namespace ash
+
+#endif  // ASH_MUS_SCREEN_MUS_H_