[Chrome OS MD] Draw a 1px separator between 2 tray items

ash/common/system/view_observer.h

+// Copyright (c) 2012 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_COMMON_SYSTEM_VIEW_OBSERVER_H_
+#define ASH_COMMON_SYSTEM_VIEW_OBSERVER_H_
+
+#include "ash/ash_export.h"
+#include "base/macros.h"
+#include "ui/views/widget/widget.h"
+
+namespace ash {
+class TrayBackgroundView;
+
+class ASH_EXPORT ViewObserver {

[varkha, 2016/07/26 23:10:15]

This name seems too generic. Maybe TrayBackgroundViewVisibilityObserver?

+ public:
+  ViewObserver() {}
+
+  // Called when |tray| visibility changes.
+  virtual void OnVisibilityChange(TrayBackgroundView* tray) {}
+
+  DISALLOW_COPY_AND_ASSIGN(ViewObserver);
+};
+
+}  // namespace ash
+
+#endif  // ASH_COMMON_SYSTEM_VIEW_OBSERVER_H_