[proxies] Implement spec change to consistency check in [[SetPrototypeOf]].

test/mjsunit/harmony/proxies-set-prototype-of.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 --harmony-reflect
 
 var target = { target: 1 };
 target.__proto__ = {};
 var handler = { handler: 1 };
 var proxy = new Proxy(target, handler);
@@ skipping 102 matching lines @@
   assertEquals([3], Reflect.getPrototypeOf(target));
 
   // Setting the prototype of a non-extensible target is fine if the prototype
   // doesn't change.
   delete handler.getPrototypeOf;
   Reflect.setPrototypeOf(proxy2, prototype);
   // Changing the prototype will throw.
   prototype = [5];
   assertThrows(() => {Reflect.setPrototypeOf(proxy2, prototype)}, TypeError);
 })();
+
+(function testProxyTrapReturnsFalse() {
+  var handler = {};
+  handler.setPrototypeOf = () => false;
+  var target = new Proxy({}, {isExtensible: () => assertUnreachable()});
+  var object = new Proxy(target, handler);
+  assertFalse(Reflect.setPrototypeOf(object, {}));
+})();