Bug 1574383 [wpt PR 18482] - Update interfaces/layout-instability.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/layout-instability.idl (#18482)

Source: https://github.com/tidoust/reffy-reports/blob/48ca519/whatwg/idl/layout-instability.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/572612386
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wpt-commits: cc4f94eff13d107d10655f3e906c2240b81b899f
wpt-pr: 18482

testing/web-platform/tests/interfaces/layout-instability.idl

@@ -7,4 +7,5 @@ interface LayoutShift : PerformanceEntry {
   readonly attribute long value;
   readonly attribute boolean hadRecentInput;
   readonly attribute DOMHighResTimeStamp lastInputTime;
+  [Default] object toJSON();
 };