mash: Shell ref clean up for screen lock

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 0dd24b166e741fd3f257e86c97ddc8f82d416865..b04881eccfe881e307753e118364371c8c120416 100644
--- a/ash/public/interfaces/session_controller.mojom
+++ b/ash/public/interfaces/session_controller.mojom
@@ -151,6 +151,10 @@ interface SessionController {
   // etc. Invoked by the screen locker during initialization.
   StartLock() => (bool locked);
 
+  // Notifies ash that chrome lock animations are finished. This is the last
+  // event for locking. SessionController forwards it to PowerEventObserver.
+  NotifyChromeLockAnimationsComplete();
+
   // 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.