Introduce ash mojo interface for lock screen action handlers

ash/action_handler/action_handler_state_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_ACTION_HANDLER_ACTION_HANDLER_STATE_OBSERVER_H_
+#define ASH_ACTION_HANDLER_ACTION_HANDLER_STATE_OBSERVER_H_
+
+#include "ash/ash_export.h"
+
+#include "ash/public/interfaces/action_handler.mojom.h"
+
+namespace ash {
+
+class ActionHandlerStateObserver {
+ public:
+  virtual ~ActionHandlerStateObserver() {}
+
+  // Called when action handler state changes.
+  virtual void OnActionStateChanged(mojom::ActionHandlerAction action,
+                                    mojom::ActionHandlerState state) = 0;
+};
+
+}  // namespace ash
+
+#endif  // ASH_ACTION_HANDLER_ACTION_HANDLER_STATE_OBSERVER_H_