[Parser] Cache and clone initial AstValueFactory string_table_.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 6d35c51830795c204d86d6b0d358666869a1a711..e93a1d50b2c7aef106599d6b9c97fd57c8c625fb 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1159,7 +1159,7 @@ class Isolate {
     return cancelable_task_manager_;
   }
 
-  AstStringConstants* ast_string_constants() const {
+  const AstStringConstants* ast_string_constants() const {
     return ast_string_constants_;
   }
 
@@ -1409,7 +1409,7 @@ class Isolate {
   std::unique_ptr<CodeEventDispatcher> code_event_dispatcher_;
   FunctionEntryHook function_entry_hook_;
 
-  AstStringConstants* ast_string_constants_;
+  const AstStringConstants* ast_string_constants_;
 
   interpreter::Interpreter* interpreter_;