mash: Run pre-unlock animation via SessionController

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 a3663657d4f8bcbf822c821e603f238a8af59790..6a2e43834d664657e5f658049c9f301516819130 100644
--- a/ash/public/interfaces/session_controller.mojom
+++ b/ash/public/interfaces/session_controller.mojom
@@ -134,6 +134,11 @@ interface SessionController {
   // 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-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() => ();
 };
 
 // Interface for ash to request session service from its client (e.g. Chrome).