Clean up forward declarations in WebKit/Source

Source/modules/encryptedmedia/MediaKeySession.h

 /*
  * Copyright (C) 2013 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 20 matching lines @@
 #include "core/dom/DOMArrayPiece.h"
 #include "core/dom/DOMException.h"
 #include "modules/EventTargetModules.h"
 #include "platform/Timer.h"
 #include "platform/heap/Handle.h"
 #include "public/platform/WebContentDecryptionModuleSession.h"
 
 namespace blink {
 
 class GenericEventQueue;
-class MediaKeyError;
 class MediaKeys;
-class WebString;
 
 // References are held by JS only. However, even if all JS references are
 // dropped, it won't be garbage collected until close event received or
 // MediaKeys goes away (as determined by the validity of a WeakPtr). This allows
 // the CDM to continue to fire events for this session, as long as the session
 // is open.
 //
 // WeakPtr<MediaKeys> is used instead of having MediaKeys and MediaKeySession
 // keep references to each other, and then having to inform the other object
 // when it gets destroyed.
@@ skipping 75 matching lines @@
     typedef ScriptPromiseProperty<Member<MediaKeySession>, ToV8UndefinedGenerator, RefPtrWillBeMember<DOMException> > ClosedPromise;
     Member<ClosedPromise> m_closedPromise;
 
     HeapDeque<Member<PendingAction> > m_pendingActions;
     Timer<MediaKeySession> m_actionTimer;
 };
 
 } // namespace blink
 
 #endif // MediaKeySession_h