Auto merge of #11432 - ab22:11402-implement-window-alert, r=KiChjang

11402 implement window alert

Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data:
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11402

Either:
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/11432)
<!-- Reviewable:end -->

components/script/dom/webidls/Window.webidl

@@ -48,8 +48,8 @@
   //readonly attribute ApplicationCache applicationCache;
 
   // user prompts
-  //void alert();
   void alert(DOMString message);
+  void alert();
   //boolean confirm(optional DOMString message = "");
   //DOMString? prompt(optional DOMString message = "", optional DOMString default = "");
   //void print();