[json] Fix iteration over object keys in InternalizeJSONProperty.

test/mjsunit/regress/regress-4769.js

Index: test/mjsunit/regress/regress-4769.js
diff --git a/test/mjsunit/regress/regress-4769.js b/test/mjsunit/regress/regress-4769.js
new file mode 100644
index 0000000000000000000000000000000000000000..6fbd01fe140cd6185f2a73b7990568cff44103aa
--- /dev/null
+++ b/test/mjsunit/regress/regress-4769.js
@@ -0,0 +1,12 @@
+// 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.
+
+// https://bugs.chromium.org/p/v8/issues/detail?id=4769
+
+Object.getPrototypeOf([])[Symbol.iterator] = () => assertUnreachable();
+
+JSON.stringify({foo: [42]});
+JSON.stringify({foo: [42]}, []);
+JSON.stringify({foo: [42]}, undefined, ' ');
+JSON.stringify({foo: [42]}, [], ' ');