Introduce ash mojo interface for lock screen action handlers

ash/tray_action/tray_action_observer.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_TRAY_ACTION_TRAY_ACTION_OBSERVER_H_
+#define ASH_TRAY_ACTION_TRAY_ACTION_OBSERVER_H_
+
+#include "ash/public/interfaces/tray_action.mojom.h"
+
+namespace ash {
+
+class TrayActionObserver {
+ public:
+  virtual ~TrayActionObserver() {}
+
+  // Called when action handler state changes.
+  virtual void OnLockScreenNoteStateChanged(mojom::TrayActionState state) = 0;
+};
+
+}  // namespace ash
+
+#endif  // ASH_TRAY_ACTION_TRAY_ACTION_OBSERVER_H_