[runtime] [proxy] Fix Object.prototype.propertyIsEnumerable to support proxies

test/mjsunit/harmony/proxies-property-is-enumerable.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: --harmony-proxies
+
+var handler = {};
+var target = { a: 1 };
+var proxy = new Proxy(target, handler);
+
+assertTrue(target.propertyIsEnumerable('a'));
+assertTrue(proxy.propertyIsEnumerable('a'));
+assertFalse(target.propertyIsEnumerable('b'));
+assertFalse(proxy.propertyIsEnumerable('b'));
+
+handler.getOwnPropertyDescriptor = function(target, prop) {
+  return { configurable: true, enumerable: true, value: 10 };
+}
+assertTrue(target.propertyIsEnumerable('a'));
+assertTrue(proxy.propertyIsEnumerable('a'));
+assertFalse(target.propertyIsEnumerable('b'));
+assertTrue(proxy.propertyIsEnumerable('b'));
+
+handler.getOwnPropertyDescriptor = function(target, prop) {
+  return { configurable: true, enumerable: false, value: 10 };
+}
+assertTrue(target.propertyIsEnumerable('a'));
+assertFalse(proxy.propertyIsEnumerable('a'));
+assertFalse(target.propertyIsEnumerable('b'));
+assertFalse(proxy.propertyIsEnumerable('b'));