Use correct theme for the off icon of NightLight, fix tooltip and a11y

ash/system/night_light/night_light_toggle_button.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_SYSTEM_NIGHT_LIGHT_NIGHT_LIGHT_TOGGLE_BUTTON_H_
+#define ASH_SYSTEM_NIGHT_LIGHT_NIGHT_LIGHT_TOGGLE_BUTTON_H_
+
+#include "ash/system/tray/system_menu_button.h"
+#include "base/macros.h"
+
+namespace ash {
+
+// The NightLight toggle button in the system tray.

[Daniel Erat, 2017/05/25 17:59:51]

nit: "Night Light" instead throughout? that's how i've seen it referred to in
bugs and docs, at least.

[afakhry, 2017/05/25 18:03:31]

I've been consistently calling it NightLight in all comments in ash/. If you
still wish that I change all the comments everywhere, can I do it in a separate
CL?

+class NightLightToggleButton : public SystemMenuButton {
+ public:
+  explicit NightLightToggleButton(views::ButtonListener* listener);
+  ~NightLightToggleButton() override = default;
+
+  // Updates the icon and its style based on the status of NightLight.
+  void Update();
+
+ private:
+  // views::View:
+  void GetAccessibleNodeData(ui::AXNodeData* node_data) override;
+
+  DISALLOW_COPY_AND_ASSIGN(NightLightToggleButton);
+};
+
+}  // namespace ash
+
+#endif  // ASH_SYSTEM_NIGHT_LIGHT_NIGHT_LIGHT_TOGGLE_BUTTON_H_