[es6] Introduce a dedicated JSIteratorResult type.

src/heap/objects-visiting.cc

Index: src/heap/objects-visiting.cc
diff --git a/src/heap/objects-visiting.cc b/src/heap/objects-visiting.cc
index 28a68eec82e0013dd0562407df22ebdc2bbe963c..6660d42e810105d627a77bc8c91c7d36f227bed8 100644
--- a/src/heap/objects-visiting.cc
+++ b/src/heap/objects-visiting.cc
@@ -137,6 +137,7 @@ StaticVisitorBase::VisitorId StaticVisitorBase::GetVisitorId(
     case JS_MESSAGE_OBJECT_TYPE:
     case JS_SET_ITERATOR_TYPE:
     case JS_MAP_ITERATOR_TYPE:
+    case JS_ITERATOR_RESULT_TYPE:
       return GetVisitorIdForSize(kVisitJSObject, kVisitJSObjectGeneric,
                                  instance_size, has_unboxed_fields);
 
@@ -226,6 +227,7 @@ void HeapObject::IterateBody(InstanceType type, int object_size,
     case JS_MAP_TYPE:
     case JS_SET_ITERATOR_TYPE:
     case JS_MAP_ITERATOR_TYPE:
+    case JS_ITERATOR_RESULT_TYPE:
     case JS_WEAK_MAP_TYPE:
     case JS_WEAK_SET_TYPE:
     case JS_REGEXP_TYPE: