Merged: [runtime] Mark old JSGlobalProxy's map as unstable when an iframe navigates away.

test/mjsunit/regress/regress-crbug-683667.js

+// Copyright 2017 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.
+
+// Flags: --expose-gc --verify-heap
+
+var realm = Realm.create();
+var g = Realm.global(realm);
+var obj = {x: 0, g: g};
+
+// Navigation will replace JSGlobalObject behind the JSGlobalProxy g and
+// therefore will change the g's map. The old map must be marked as non-stable.
+Realm.navigate(realm);
+gc();