Allow plugins from a parent document to be used in an iframe (#12208)

* Fix cross iframe isPlainObject check

* Update changelog

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Co-authored-by: Jordan Pittman <jordan@cryptica.me>

src/util/isPlainObject.js

@@ -4,5 +4,5 @@ export default function isPlainObject(value) {
   }
 
   const prototype = Object.getPrototypeOf(value)
-  return prototype === null || prototype === Object.prototype
+  return prototype === null || Object.getPrototypeOf(prototype) === null
 }