[builtins] move String.prototype[@@iterator] to C++ builtin

src/factory.h

Index: src/factory.h
diff --git a/src/factory.h b/src/factory.h
index fc287694e66b4ce91832d0f6223d051311a8c71e..30361959f65ac856ba5b3db1aacbe75a810427ee 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -176,6 +176,9 @@ class Factory final {
   MUST_USE_RESULT MaybeHandle<String> NewStringFromTwoByte(
       const ZoneVector<uc16>* str, PretenureFlag pretenure = NOT_TENURED);
 
+  Handle<JSStringIterator> NewJSStringIterator();
+  Handle<JSStringIterator> NewJSStringIterator(Handle<String> string);
+
   // Allocates an internalized string in old space based on the character
   // stream.
   Handle<String> NewInternalizedStringFromUtf8(Vector<const char> str,
@@ -504,6 +507,8 @@ class Factory final {
   Handle<JSDataView> NewJSDataView(Handle<JSArrayBuffer> buffer,
                                    size_t byte_offset, size_t byte_length);
 
+  Handle<JSIteratorResult> NewJSIteratorResult(Handle<Object> value, bool done);
+
   Handle<JSMap> NewJSMap();
   Handle<JSSet> NewJSSet();