Add minimal implementation of unprefixed MediaSource API that has feature parity with prefixed API

Source/core/dom/EventTarget.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  *           (C) 2007, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ skipping 50 matching lines @@
     class NamedFlow;
     class Node;
     class Notification;
     class SVGElementInstance;
     class ScriptExecutionContext;
     class ScriptProcessorNode;
     class SharedWorker;
     class SharedWorkerContext;
     class TextTrack;
     class TextTrackCue;
-    class WebKitMediaSource;
-    class WebKitSourceBufferList;
     class WebSocket;
     class Worker;
     class XMLHttpRequest;
     class XMLHttpRequestUpload;
 
     typedef int ExceptionCode;
 
     struct FiringEventIterator {
         FiringEventIterator(const AtomicString& eventType, size_t& iterator, size_t& end)
             : eventType(eventType)
@@ skipping 109 matching lines @@
     {
         EventTargetData* d = eventTargetData();
         if (!d)
             return false;
         return d->eventListenerMap.contains(eventType);
     }
 
 } // namespace WebCore
 
 #endif // EventTarget_h