Simplify invokeCallback() and support void return values for IDL callbacks

Source/core/html/VoidCallback.h

 /*
  * Copyright (C) 2007 Apple Inc. All rights reserved.
  *
  * 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 13 matching lines @@
  */
 
 #ifndef VoidCallback_h
 #define VoidCallback_h
 
 namespace WebCore {
 
 class VoidCallback {
 public:
     virtual ~VoidCallback() { }
-    virtual bool handleEvent() = 0;
+    virtual void handleEvent() = 0;
 };
 
 } // namespace WebCore
 
 #endif