Map::ReconfigureProperty() should mark map as unstable when it returns a different map.

test/mjsunit/regress/regress-crbug-502930.js

+// Copyright 2015 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 accessor_to_data_case = (function() {
+  var v = {};
+  Object.defineProperty(v, "foo", { get: function() { return 42; }, configurable: true});
+
+  var obj = {};
+  obj["boom"] = v;
+
+  Object.defineProperty(v, "foo", { value: 0, writable: true, configurable: true });
+  return obj;
+})();
+
+
+var data_to_accessor_case = (function() {
+  var v = {};
+  Object.defineProperty(v, "bar", { value: 0, writable: true, configurable: true });
+
+  var obj = {};
+  obj["bam"] = v;
+
+  Object.defineProperty(v, "bar", { get: function() { return 42; }, configurable: true});
+  return obj;
+})();