Bug 261528

Steps to reproduce: Open JS console: ``` let a = { "a": 12 } structuredClone(a.proto) let b = new RegExp(".", "") structuredClone(b.proto) ``` Actual results: The prototype of the ordinary object a is cloned and printed to the console. A DOM Exception is thrown when trying to clone the RegExp.prototype object from b. Expected results: The spec steps for structured serialize internal (html.spec.whatwg.org/multipage/structured-data.html#structuredserializeinternal) don't seem to preclude serializing intrinsic object prototypes: In step 21, the algorithm precludes any objects with funky internal slots Otherwise, if value has any internal slot other than [[Prototype]] or [[Extensible]], then throw a "DataCloneError" DOMException. If we look at the ES spec for RegExp Prototype: tc39.es/ecma262/#sec-properties-of-the-regexp-prototype-object It says that that object: is %RegExp.prototype%. is an ordinary object. is not a RegExp instance and does not have a [[RegExpMatcher]] internal slot or any of the other internal slots of RegExp instance objects. has a [[Prototype]] internal slot whose value is %Object.prototype%. Which suggests to me that it should fall through to the next step, step 23, which says: Otherwise, if value is an exotic object and value is not the %Object.prototype% intrinsic object associated with any realm, then throw a "DataCloneError" DOMException. Since %RegExp.prototype% is an ordinary object, it's not exotic, and so it should be cloneable as any other object. Chromium does this per the spec, and Gecko does not. Gecko bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1853050