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

Source/core/dom/StringCallback.h

 /*
  * Copyright (C) 2010 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 22 matching lines @@
 
 #include "wtf/Forward.h"
 
 namespace WebCore {
 
 class ExecutionContext;
 
 class StringCallback {
 public:
     virtual ~StringCallback() { }
-    virtual bool handleEvent(const String& data) = 0;
+    virtual void handleEvent(const String& data) = 0;
 
     // Helper to post callback task.
     static void scheduleCallback(PassOwnPtr<StringCallback>, ExecutionContext*, const String& data);
 };
 
 } // namespace WebCore
 
 #endif // StringCallback_h