[heap] Prevent direct access to ExternalStringTable.

src/extensions/externalize-string-extension.cc

Index: src/extensions/externalize-string-extension.cc
diff --git a/src/extensions/externalize-string-extension.cc b/src/extensions/externalize-string-extension.cc
index 3eaa70e515ee1f2b5c16f6833ef5f775ee347a25..9241e9f4de87e76abd86d739a18481528b61a569 100644
--- a/src/extensions/externalize-string-extension.cc
+++ b/src/extensions/externalize-string-extension.cc
@@ -98,7 +98,7 @@ void ExternalizeStringExtension::Externalize(
     result = string->MakeExternal(resource);
     if (result) {
       i::Isolate* isolate = reinterpret_cast<i::Isolate*>(args.GetIsolate());
-      isolate->heap()->external_string_table()->AddString(*string);
+      isolate->heap()->RegisterExternalString(*string);
     }
     if (!result) delete resource;
   } else {
@@ -109,7 +109,7 @@ void ExternalizeStringExtension::Externalize(
     result = string->MakeExternal(resource);
     if (result) {
       i::Isolate* isolate = reinterpret_cast<i::Isolate*>(args.GetIsolate());
-      isolate->heap()->external_string_table()->AddString(*string);
+      isolate->heap()->RegisterExternalString(*string);
     }
     if (!result) delete resource;
   }