Don't use normalized map cache for prototype maps

test/mjsunit/regress/regress-crbug-477924.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 dummy = new ReferenceError("dummy");
+
+constructors = [ ReferenceError, TypeError];
+
+for (var i in constructors) {
+  constructors[i].prototype.__defineGetter__("stack", function() {});
+}
+
+for (var i in constructors) {
+  var obj = new constructors[i];
+  obj.toString();
+}