[Night Light] CL2: Ash and system tray work

ash/system/night_light/tray_night_light.h

Index: ash/system/night_light/tray_night_light.h
diff --git a/ash/system/night_light/tray_night_light.h b/ash/system/night_light/tray_night_light.h
new file mode 100644
index 0000000000000000000000000000000000000000..5a38d3f9f869670e1ce779ebf4d197f84b5eeebc
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+++ b/ash/system/night_light/tray_night_light.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_NIGHT_LIGHT_TRAY_NIGHT_LIGHT_H_
+#define ASH_SYSTEM_NIGHT_LIGHT_TRAY_NIGHT_LIGHT_H_
+
+#include "ash/system/night_light/night_light_controller.h"
+#include "ash/system/tray/tray_image_item.h"
+#include "base/macros.h"
+
+namespace ash {
+
+// Shows the NightLight icon in the system tray whenever NightLight is enabled
+// and active.
+class TrayNightLight : public TrayImageItem,
+                       public NightLightController::Observer {
+ public:
+  explicit TrayNightLight(SystemTray* system_tray);
+  ~TrayNightLight() override;
+
+  // ash::NightLightController::Observer:
+  void OnNightLightEnabledChanged(bool enabled) override;
+
+  // ash::TrayImageItem:
+  bool GetInitialVisibility() override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(TrayNightLight);
+};
+
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_NIGHT_LIGHT_TRAY_NIGHT_LIGHT_H_