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

src/factory.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/factory.h"
 
 #include "src/accessors.h"
 #include "src/allocation-site-scopes.h"
 #include "src/ast/ast.h"
 #include "src/base/bits.h"
@@ skipping 1835 matching lines @@
   array->set_elements(*elms);
   array->set_length(Smi::FromInt(length));
 }
 
 Handle<JSModuleNamespace> Factory::NewJSModuleNamespace() {
   Handle<Map> map = isolate()->js_module_namespace_map();
   Handle<JSModuleNamespace> module_namespace(
       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;
 }
 
 Handle<JSGeneratorObject> Factory::NewJSGeneratorObject(
     Handle<JSFunction> function) {
   DCHECK(IsResumableFunction(function->shared()->kind()));
   JSFunction::EnsureHasInitialMap(function);
   Handle<Map> map(function->initial_map());
   DCHECK_EQ(JS_GENERATOR_OBJECT_TYPE, map->instance_type());
   CALL_HEAP_FUNCTION(
@@ skipping 935 matching lines @@
     Handle<AccessorInfo> prototype =
         Accessors::FunctionPrototypeInfo(isolate(), rw_attribs);
     Descriptor d = Descriptor::AccessorConstant(
         Handle<Name>(Name::cast(prototype->name())), prototype, rw_attribs);
     map->AppendDescriptor(&d);
   }
 }
 
 }  // namespace internal
 }  // namespace v8