Use correct order when defining length/name/prototype on legacy facto…

…ry functions.

Implements whatwg/webidl#914 for legacy factory functions.
https://bugzilla.mozilla.org/show_bug.cgi?id=1629803 is the bug that would fix
this in SpiderMonkey, working around that for now.

Differential Revision: https://phabricator.services.mozilla.com/D205805

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1887721
gecko-commit: 0594801ef8e281f5be66076d27a72c5c8c9496e9
gecko-reviewers: saschanaz

webidl/ecmascript-binding/legacy-factory-function-builtin-properties.window.js

@@ -0,0 +1,6 @@
+"use strict";
+
+test(() => {
+  const ownPropKeys = Reflect.ownKeys(Image).slice(0, 3);
+  assert_array_equals(ownPropKeys, ["length", "name", "prototype"]);
+}, 'Legacy factory function property enumeration order of "length", "name", and "prototype"');