Fix corner-case behavior of JSObject::SetPrototype.

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

 // Copyright 2014-2015 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 118 matching lines @@
 }
 TestSetPrototypeToNull();
 
 
 function TestSetPrototypeOfNonExtensibleObject() {
   var objects = getObjects();
   var proto = {};
   for (var i = 0; i < objects.length; i++) {
     var object = objects[i];
     Object.preventExtensions(object);
+    // Setting the current prototype must succeed.
+    assertTrue(Reflect.setPrototypeOf(object, Object.getPrototypeOf(object)));
+    // Setting any other must fail.
     assertFalse(Reflect.setPrototypeOf(object, proto));
   }
 }
 TestSetPrototypeOfNonExtensibleObject();
 
 
 function TestSetPrototypeCyclic() {
   var objects = [
     Object.prototype, {},
     Array.prototype, [],
@@ skipping 23 matching lines @@
   assertEquals(object.y, 'old y');
 
   var newProto = {
     x: 'new x'
   };
   assertTrue(Reflect.setPrototypeOf(object, newProto));
   assertEquals(object.x, 'new x');
   assertFalse('y' in object);
 }
 TestLookup();