cros: Add enum to helper function that I forgot to update.

chromeos_wm_ipc_enums.h

Index: chromeos_wm_ipc_enums.h
diff --git a/chromeos_wm_ipc_enums.h b/chromeos_wm_ipc_enums.h
index 1553757f9b74af66d25547358cb22ce44dc74917..eeb25c0eb2e6c75c95362b6e456b1915a20600c3 100644
--- a/chromeos_wm_ipc_enums.h
+++ b/chromeos_wm_ipc_enums.h
@@ -300,6 +300,8 @@ inline const char* WmIpcMessageTypeToString(WmIpcMessageType type) {
       return "WM_NOTIFY_SHUTTING_DOWN";
     case WM_IPC_MESSAGE_WM_NOTIFY_POWER_BUTTON_STATE:
       return "WM_NOTIFY_POWER_BUTTON_STATE";
+    case WM_IPC_MESSAGE_WM_NOTIFY_SIGNING_OUT:
+      return "WM_NOTIFY_SIGNING_OUT";
     default:
       return "INVALID";
   }