cros: Use SessionController for lock starting code

ash/public/interfaces/session_controller.mojom

 // Copyright 2016 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";
 import "ui/gfx/image/mojo/image.mojom";
 
 // Matches session_manager::SessionState.
@@ skipping 125 matching lines @@
   // its meta data (e.g. name, avatar) is changed. There is no method to remove
   // a user session because ash/chrome does not support that. All users are
   // logged out at the same time.
   UpdateUserSession(UserSession user_session);
 
   // Sets the order of user sessions. The order is keyed by the session id.
   // Currently, session manager set a LRU order with the first one being the
   // active user session.
   SetUserSessionOrder(array<uint32> user_session_ids);
 
+  // Runs the pre-lock animation to start locking. The call returns after
+  // the post-lock animation finishes and the system is fully locked. When

[Tom Sepez, 2017/04/20 01:25:00]

Can something go wrong and this fail to lock? Do we need status return?

[xiyuan, 2017/04/20 15:14:40]

LockStateController has a |lock_fail_timer_| to handle this case. The flow is
roughly like this:
1. LockStateController runs pre-lock animation and
   starts |lock_fail_timer_|
   (triggered by this call as ScreenLocker responses to the
    lock request);
2. Meanwhile, ScreenLocker loads the webui and sets LOCKED state
   when the webui loads and finishes initialization;
3. If LOCKED states comes before |lock_fail_timer_| fires,
   LockStateController runs post-lock animation and the callback
   will be called;
4. If |lock_fail_timer_| fires first, we bail (sign out the
   current user and go back to login screen);

This CL changes the lock starting part (Step 1) from a directly
LockStateController call to this mojo call. The rest of the flow should still
stand.

And, a status is necessary to handle the test failures and mojo DCHECK when
|lock_fail_timer_| fires. Thanks for the suggestion. Will add a bool |locked|.

+  // it returns, screen locker runs the post lock jobs such as a11y
+  // announcement etc. Invoked by the screen locker during initialization.
+  RunLockAnimation() => ();
+
   // Runs the pre-unlock animation. Invoked by the screen locker before
   // dismissing. When the mojo call returns, screen locker takes that as a
   // signal of finished unlock animation and dismisses itself.
   RunUnlockAnimation() => ();
 
   // Notifies that chrome is terminating.
   NotifyChromeTerminating();
 };
 
 // Interface for ash to request session service from its client (e.g. Chrome).
 interface SessionControllerClient {
   // Requests to lock screen.
   RequestLockScreen();
 
   // Switch to the active user with |account_id| (if the user has already signed
   // in).
   SwitchActiveUser(signin.mojom.AccountId account_id);
 
   // Switch the active user to the next or previous user.
   CycleActiveUser(CycleUserDirection direction);
 };