Introduce ash mojo interface for lock screen action handlers

ash/public/interfaces/tray_action.mojom

Index: ash/public/interfaces/tray_action.mojom
diff --git a/ash/public/interfaces/tray_action.mojom b/ash/public/interfaces/tray_action.mojom
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+// 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.
+
+module ash.mojom;
+
+// An action handler state.
+enum TrayActionState {
+  // The action cannot be handled - due to no client being set, the client not
+  // supporting the action, user session not being locked etc.
+  kNotAvailable,
+  
+  // The client supports the action and is not currently handling the action.
+  kAvailable,
+
+  // The client received the request for the action and it is launching the
+  // flow to handle it.
+  kLaunching,
+
+  // The client is currently handling the action.
+  kActive,
+
+  // The client is currently handling the action, but in background (e.g. the
+  // handler window for the lock screen action was moved to the background and
+  // appears underneath the login user pods).
+  kBackground,
+};
+
+// Used by a client (e.g. Chrome) to set up a handler for a tray action, and
+// notify ash on the action handler's state changes. A tray action is one of
+// predefined actions (currently only the "new note on lock screen" action is
+// supported) that appear as an ash status area button if the client declares
+// the action as available. Clicking the button invokes a client method that
+// requests the action associated with the button to be handled.
+interface TrayAction {
+  // Sets the client to be used to handle action requests.
+  // |lock_screen_note_state|: The current lock screen note action state
+  //     associated with the client.
+  SetClient(TrayActionClient client, TrayActionState lock_screen_note_state);
+
+  // Updates action state for the lock screen note action. If called with no
+  // client set, the state change will not take effect until a client is set.
+  // Null client is equivalent to kNotAvailable state.
+  UpdateLockScreenNoteState(TrayActionState state);
+};
+
+// Used by ash to request Chrome to handle an action.
+interface TrayActionClient {
+  // Requests a lock screen note action to be handled.
+  RequestNewLockScreenNote();
+};