Fix ArrayLengthSetter to not throw on non-extensible receivers.

test/mjsunit/regress/regress-mask-array-length.js

Index: test/mjsunit/regress/regress-mask-array-length.js
diff --git a/test/mjsunit/regress/regress-349870.js b/test/mjsunit/regress/regress-mask-array-length.js
similarity index 65%
copy from test/mjsunit/regress/regress-349870.js
copy to test/mjsunit/regress/regress-mask-array-length.js
index 72df05524bf1ccbcc8e4201512238e6f99e3fdea..bd87e7c5db10d92da62d131ed8b8d50e2f8e5a78 100644
--- a/test/mjsunit/regress/regress-349870.js
+++ b/test/mjsunit/regress/regress-mask-array-length.js
@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-var r = /x/;
-Object.freeze(r);
-r.compile("x");
+var a = [];
+var o = {
+  __proto__: a
+};
+Object.preventExtensions(o);
+o.length = 'abc';