According to spec https://tc39.es/ecma262/#sec-object.preventextensions, the commit 8e0ef9b9 is missing the last step when object is proxy, it needs to return the object.
var proxy = new Proxy({}, {});
var object = Object.preventExtensions(proxy);
proxy === object; // should be true

Also, add mjsunit test.

Bug: v8:6664
Change-Id: Ic3688519539f8903ee0bc7e885905a86d195a4db
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1668443
Commit-Queue: Z Nguyen-Huu <duongn@microsoft.com>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62393}