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 _____

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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();

components/script/dom/window.rs

@@ -430,6 +430,11 @@ pub fn base64_atob(input: DOMString) -> Fallible<DOMString> {
 }
 
 impl WindowMethods for Window {
+    // https://html.spec.whatwg.org/multipage/#dom-alert
+    fn Alert_(&self) {
+        self.Alert(DOMString::new());
+    }
+
     // https://html.spec.whatwg.org/multipage/#dom-alert
     fn Alert(&self, s: DOMString) {
         // Right now, just print to the console