[json] fix encoding change for two-byte gap strings.

test/mjsunit/regress/regress-crbug-613570.js

Index: test/mjsunit/regress/regress-crbug-613570.js
diff --git a/test/message/for-of-throw-in-body.js b/test/mjsunit/regress/regress-crbug-613570.js
similarity index 64%
copy from test/message/for-of-throw-in-body.js
copy to test/mjsunit/regress/regress-crbug-613570.js
index 38b27f38632520e292f66b7ab451df00c2c03b68..3cd9857761fe5de42ef07f2f2a151f6b4edf0334 100644
--- a/test/message/for-of-throw-in-body.js
+++ b/test/mjsunit/regress/regress-crbug-613570.js
@@ -2,4 +2,5 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-for (var x of [1, 2, 3]) { throw 42 }
+assertEquals("[\n\u26031,\n\u26032\n]",
+             JSON.stringify([1, 2], null, "\u2603"));