[ES6 modules] Warn about ModuleScript::CreateParseError usage

Bug: None
Change-Id: I16c48291147a71dd119983811e45e3a268ef963e
Reviewed-on: https://chromium-review.googlesource.com/c/1343411Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613226}

third_party/blink/renderer/core/script/module_script.h

@@ -54,8 +54,14 @@ class CORE_EXPORT ModuleScript final : public Script, public NameClient {
   ScriptModule Record() const;
   bool HasEmptyRecord() const;
 
+  // Note: ParseError-related methods should only be used from ModuleTreeLinker
+  //       or unit tests. You probably want to check |*ErrorToRethrow*()|
+  //       instead.
+
   void SetParseErrorAndClearRecord(ScriptValue error);
   bool HasParseError() const { return !parse_error_.IsEmpty(); }
+
+  // CreateParseError() retrieves |parse_error_| as a ScriptValue.
   ScriptValue CreateParseError() const;
 
   void SetErrorToRethrow(ScriptValue error);