Implements a display scale tray setting behind a chrome switch

ash/system/display_scale/tray_scale.h

Index: ash/system/display_scale/tray_scale.h
diff --git a/ash/system/display_scale/tray_scale.h b/ash/system/display_scale/tray_scale.h
new file mode 100644
index 0000000000000000000000000000000000000000..64d3277b71c58b5211da60b2c05ee04c11d2b7a7
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+++ b/ash/system/display_scale/tray_scale.h
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+// 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_SYSTEM_DISPLAY_SCALE_TRAY_SCALE_H_
+#define ASH_SYSTEM_DISPLAY_SCALE_TRAY_SCALE_H_
+
+#include <stdint.h>
+
+#include "ash/ash_export.h"
+#include "ash/system/tray/tray_image_item.h"
+#include "base/macros.h"
+
+namespace ash {
+namespace tray {
+class ScaleView;
+class ScaleDetailedView;
+}  // namespace tray
+
+// The system tray item for display scale.
+class ASH_EXPORT TrayScale : public SystemTrayItem {
+ public:
+  explicit TrayScale(SystemTray* system_tray);
+  ~TrayScale() override;
+
+ private:
+  // Overridden from SystemTrayItem.
+  views::View* CreateDefaultView(LoginStatus status) override;
+  views::View* CreateDetailedView(LoginStatus status) override;
+  void DestroyDefaultView() override;
+  void DestroyDetailedView() override;
+  bool ShouldShowShelf() const override;
+
+  tray::ScaleView* scale_view_;
+
+  tray::ScaleDetailedView* scale_detail_view_;
+
+  DISALLOW_COPY_AND_ASSIGN(TrayScale);
+};
+
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_DISPLAY_SCALE_TRAY_SCALE_H_