Fix prototype registration upon SlowToFast migration

test/mjsunit/regress/regress-crbug-513602.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.
+
+function Parent() {}
+
+function Child() {}
+Child.prototype = new Parent();
+var child = new Child();
+
+function crash() {
+  return child.__proto__;
+}
+
+crash();
+crash();
+
+// Trigger a fast->slow->fast dance of Parent.prototype's map...
+Parent.prototype.__defineSetter__("foo", function() { print("A"); });
+Parent.prototype.__defineSetter__("foo", function() { print("B"); });
+// ...and collect more type feedback.
+crash();
+
+// Now modify the prototype chain. The right cell fails to get invalidated.
+delete Object.prototype.__proto__;
+crash();