Fix corner-case behavior of JSObject::SetPrototype.

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

Index: test/mjsunit/harmony/set-prototype-of.js
diff --git a/test/mjsunit/harmony/set-prototype-of.js b/test/mjsunit/harmony/set-prototype-of.js
index e92a0c37bc3de9eb3b4750b0474581319061304e..7406fb54bf225e25e4f0cf7ae5c3f60ed13ae4f6 100644
--- a/test/mjsunit/harmony/set-prototype-of.js
+++ b/test/mjsunit/harmony/set-prototype-of.js
@@ -131,6 +131,9 @@ function TestSetPrototypeOfNonExtensibleObject() {
   for (var i = 0; i < objects.length; i++) {
     var object = objects[i];
     Object.preventExtensions(object);
+    // Setting the current prototype must succeed.
+    Object.setPrototypeOf(object, Object.getPrototypeOf(object));
+    // Setting any other must fail.
     assertThrows(function() {
       Object.setPrototypeOf(object, proto);
     }, TypeError);