cros: Make sure views-based lock screen is destroyed after it is dismissed.

ash/public/interfaces/lock_screen.mojom

 // 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;
 
 import "components/signin/public/interfaces/account_id.mojom";
 
 // Allows clients (e.g. Chrome browser) to control the ash lock screen.
 interface LockScreen {
   // Sets the client interface.
   SetClient(LockScreenClient client);
 
+  // Displays the lock screen. |did_show| is true iff the lock UI was
+  // successfully displayed.
+  ShowLockScreen() => (bool did_show);
+
   // Requests to show error message in the ash lock screen.
   // TODO(xiaoyinh): login_attempts is probabaly not needed from chrome,
   // remove it when we start to count the login attempts in ash lock screen.
   // |login_attempts|: The number of the login authentication attempts.
   // |error_text|:     The error text to be shown in lock screen.
   // |help_link_text|: The help link to be shown in lock screen.
   // |help_topic_id|:  The id of the help app topic regarding this error.
   ShowErrorMessage(int32 login_attempts,
                    string error_text,
                    string help_link_text,
                    int32 help_topic_id);
 
   // Requests to close any displayed error messages in ash lock screen.
   ClearErrors();
 };
 
 // Allows ash lock screen to control a client (e.g. Chrome browser). Requests
 // often involve preferences or talk to cryptohome that is not available to ash.
 interface LockScreenClient {
   // Request user authentication in chrome.
   // If auth succeeds:
   // chrome will hide the lock screen and clear any displayed error messages.
   // If auth fails:
   // chrome will request lock screen to show error messages.
   // |account_id|:           The account id of the user we are authenticating.
   // |hashed_password|:      The hashed password of the user.
   // |authenticated_by_pin|: True if we are using pin to authenticate.
+  //
+  // The result will be set to true if auth was successful, false if not.
   AuthenticateUser(signin.mojom.AccountId account_id,
                    string hashed_password,
-                   bool authenticated_by_pin);
+                   bool authenticated_by_pin) => (bool auth_success);
 };