Bug 1574383 [wpt PR 18482] - Update wpt metadata, a=testonly

wpt-pr: 18482
wpt-type: metadata

testing/web-platform/meta/layout-instability/idlharness.window.js.ini

@@ -26,3 +26,6 @@
   [LayoutShift interface: existence and properties of interface prototype object's @@unscopables property]
     expected: FAIL
 
+  [LayoutShift interface: operation toJSON()]
+    expected: FAIL
+