Sync the alert() IDL with the spec

Source/core/frame/DOMWindow.h

 /*
  * Copyright (C) 2006, 2007, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 141 matching lines @@
         void print();
         void stop();
 
         PassRefPtrWillBeRawPtr<DOMWindow> open(const String& urlString, const AtomicString& frameName, const String& windowFeaturesString,
             DOMWindow* callingWindow, DOMWindow* enteredWindow);
 
         typedef void (*PrepareDialogFunction)(DOMWindow*, void* context);
         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);
 
         bool find(const String&, bool caseSensitive, bool backwards, bool wrap, bool wholeWord, bool searchInFrames, bool showDialog) const;
 
         bool offscreenBuffering() const;
 
         int outerHeight() const;
         int outerWidth() const;
         int innerHeight() const;
@@ skipping 213 matching lines @@
     }
 
     inline String DOMWindow::defaultStatus() const
     {
         return m_defaultStatus;
     }
 
 } // namespace WebCore
 
 #endif // DOMWindow_h