Expose a way to make a "same-origin" realm

test/mjsunit/realm-property-access.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var r = Realm.create();
+var f = Realm.eval(r, "function f() { return this }; f()");
+assertEquals(f, Realm.global(r));
+
+// Cross-origin property access throws
+assertThrows(() => f.a, TypeError);
+assertThrows(() => { 'use strict'; f.a = 1 }, TypeError);
+
+var r2 = Realm.createAllowCrossRealmAccess();
+var f2 = Realm.eval(r2, "function f() { return this }; f()");
+assertEquals(f2, Realm.global(r2));
+
+// Same-origin property access doesn't throw
+assertEquals(undefined, f2.a);
+f2.a = 1;
+assertEquals(1, f2.a);