[modules] Fix NewJSModuleNamespace to avoid GC being able to see incomplete JSModuleNamespace.

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index 0e34a72754a49f5aabbe915cb20aa296feae3565..a2742d7defbbb808dde46224ab193c26c32f1731 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -1853,8 +1853,8 @@ Handle<JSModuleNamespace> Factory::NewJSModuleNamespace() {
       Handle<JSModuleNamespace>::cast(NewJSObjectFromMap(map)));
   FieldIndex index = FieldIndex::ForDescriptor(
       *map, JSModuleNamespace::kToStringTagFieldIndex);
-  Handle<String> to_string_value = NewStringFromAsciiChecked("Module");
-  module_namespace->FastPropertyAtPut(index, *to_string_value);
+  module_namespace->FastPropertyAtPut(index,
+                                      isolate()->heap()->Module_string());
   return module_namespace;
 }