ThinStrings: fix Factory::NewProperSubString

test/mjsunit/regress/regress-crbug-685504.js

Index: test/mjsunit/regress/regress-crbug-685504.js
diff --git a/test/mjsunit/regress/regress-crbug-685504.js b/test/mjsunit/regress/regress-crbug-685504.js
new file mode 100644
index 0000000000000000000000000000000000000000..83975bfd6dee37ffc9134424994a7a48ea1d11e1
--- /dev/null
+++ b/test/mjsunit/regress/regress-crbug-685504.js
@@ -0,0 +1,18 @@
+// 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.
+
+var v2 = 1073741823;
+var v13 = {};
+function f1(a, b) {
+  var v4 = a + b;
+  var v1 = v4.substring(20);
+  v2[v4];
+  return v1;
+}
+
+v5 = f1("abcdefghijklmnopqrstuvwxyz", "abcdefghijklmnopqrstuvwxyz");
+function f8(name, input, regexp) {
+  var v14 = input.match(regexp);
+  RegExp["$'"]}
+f8("CaptureGlobal", v5, v13, []["anama"]);