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

test/mjsunit/harmony/proxies-set-prototype-of.js

Index: test/mjsunit/harmony/proxies-set-prototype-of.js
diff --git a/test/mjsunit/harmony/proxies-set-prototype-of.js b/test/mjsunit/harmony/proxies-set-prototype-of.js
index bc60ff492c45ea126f911e27fb35ffffe81cf0ee..810c219533996fd6ab6e324f8e7072bd27a1deaf 100644
--- a/test/mjsunit/harmony/proxies-set-prototype-of.js
+++ b/test/mjsunit/harmony/proxies-set-prototype-of.js
@@ -120,3 +120,11 @@ assertEquals({a:5}, seen_prototype);
   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, {}));
+})();