[es6] Introduce a dedicated JSIteratorResult type.

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index 375c0bd3bf76f5c859be18e64d1e0dbbf2e20153..c67a3f5327483a9058fa084c85b92e5bba948741 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -475,6 +475,11 @@ class Factory final {
   Handle<JSMapIterator> NewJSMapIterator();
   Handle<JSSetIterator> NewJSSetIterator();
 
+  // Creates a new JSIteratorResult object with the arguments {value} and
+  // {done}.  Implemented according to ES6 section 7.4.7 CreateIterResultObject.
+  Handle<JSIteratorResult> NewJSIteratorResult(Handle<Object> value,
+                                               Handle<Object> done);
+
   // Allocates a Harmony proxy.
   Handle<JSProxy> NewJSProxy(Handle<Object> handler, Handle<Object> prototype);