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

src/objects-debug.cc

Index: src/objects-debug.cc
diff --git a/src/objects-debug.cc b/src/objects-debug.cc
index b860e73d150b9ce06c62b7baf54f5936d5cb341c..37f1b8c225a21a46cd96bef39ba7951dcc746b34 100644
--- a/src/objects-debug.cc
+++ b/src/objects-debug.cc
@@ -152,6 +152,9 @@ void HeapObject::HeapObjectVerify() {
     case JS_MAP_ITERATOR_TYPE:
       JSMapIterator::cast(this)->JSMapIteratorVerify();
       break;
+    case JS_STRING_ITERATOR_TYPE:
+      JSStringIterator::cast(this)->JSStringIteratorVerify();
+      break;
     case JS_WEAK_MAP_TYPE:
       JSWeakMap::cast(this)->JSWeakMapVerify();
       break;
@@ -781,6 +784,13 @@ void JSWeakMap::JSWeakMapVerify() {
   CHECK(table()->IsHashTable() || table()->IsUndefined(GetIsolate()));
 }
 
+void JSStringIterator::JSStringIteratorVerify() {
+  CHECK(IsJSStringIterator());
+  JSObjectVerify();
+  Isolate* isolate = GetIsolate();
+  CHECK(string()->IsString());
+  CHECK(index()->IsSmi() || index()->IsUndefined(isolate));
+}
 
 void JSWeakSet::JSWeakSetVerify() {
   CHECK(IsJSWeakSet());