Sync the alert() IDL with the spec

Source/core/frame/DOMWindow.h

Index: Source/core/frame/DOMWindow.h
diff --git a/Source/core/frame/DOMWindow.h b/Source/core/frame/DOMWindow.h
index 9c32d5f45e981d4a31f7cfb7649178a454bf1933..d7a40d1103f63f797c4ef1fd1bc881cf1ffc8ca1 100644
--- a/Source/core/frame/DOMWindow.h
+++ b/Source/core/frame/DOMWindow.h
@@ -159,7 +159,7 @@ enum PageshowEventPersistence {
         void showModalDialog(const String& urlString, const String& dialogFeaturesString,
             DOMWindow* callingWindow, DOMWindow* enteredWindow, PrepareDialogFunction, void* functionContext);
 
-        void alert(const String& message);
+        void alert(const String& message = String());
         bool confirm(const String& message);
         String prompt(const String& message, const String& defaultValue);