Map::ReconfigureProperty() should mark map as unstable when there is an element kind transition som…

test/mjsunit/regress/regress-crbug-485548-2.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.
+
+// Flags: --allow-natives-syntax --expose-gc
+
+var inner = new Array();
+inner.a = {x:1};
+inner[0] = 1.5;
+inner.b = {x:2};
+assertTrue(%HasFastDoubleElements(inner));
+
+function foo(o) {
+  return o.field.b.x;
+}
+
+var outer = {};
+outer.field = inner;
+foo(outer);
+foo(outer);
+foo(outer);
+%OptimizeFunctionOnNextCall(foo);
+foo(outer);
+
+// Generalize representation of field "b" of inner object.
+var v = { get x() { return 0x7fffffff; } };
+inner.b = v;
+
+gc();
+
+var boom = foo(outer);
+print(boom);
+assertEquals(0x7fffffff, boom);