Some tests, e.g. in test262, want to create a new same-origin
realm. This patch exposes a new function,
Realm.createAllowCrossRealmAccess(), which vends a new realm with
the same security token as the currently executing one.

Review-Url: https://codereview.chromium.org/1973363004
Cr-Commit-Position: refs/heads/master@{#36561}