Introduce ash mojo interface for lock screen action handlers

ash/tray_action/tray_action_observer.h

Index: ash/tray_action/tray_action_observer.h
diff --git a/ash/tray_action/tray_action_observer.h b/ash/tray_action/tray_action_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..6a8faca408fa0e4c2d44e5c7ecbf27ea7184e605
--- /dev/null
+++ b/ash/tray_action/tray_action_observer.h
@@ -0,0 +1,22 @@
+// 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_