cros: Use SessionController for lock starting code

ash/public/interfaces/session_controller.mojom

Index: ash/public/interfaces/session_controller.mojom
diff --git a/ash/public/interfaces/session_controller.mojom b/ash/public/interfaces/session_controller.mojom
index ed066cbd2a87399167ef62e0c698e3f170b7580d..ef5834401915c6fa7319bd57065b8c385e091310 100644
--- a/ash/public/interfaces/session_controller.mojom
+++ b/ash/public/interfaces/session_controller.mojom
@@ -143,6 +143,12 @@ interface SessionController {
   // 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.