Fix corner-case behavior of JSObject::SetPrototype.

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

Index: test/mjsunit/harmony/reflect-set-prototype-of.js
diff --git a/test/mjsunit/harmony/reflect-set-prototype-of.js b/test/mjsunit/harmony/reflect-set-prototype-of.js
index 95b05aacd9054080a183e6d4f3aed98592a77313..0e5554e907aca9021e0902969b52e7e9449d766b 100644
--- a/test/mjsunit/harmony/reflect-set-prototype-of.js
+++ b/test/mjsunit/harmony/reflect-set-prototype-of.js
@@ -136,6 +136,9 @@ function TestSetPrototypeOfNonExtensibleObject() {
   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));
   }
 }