Clean up forward declarations in Source/modules

Source/modules/mediastream/MediaStreamTrackSourcesCallback.h

 /*
  * Copyright (C) 2013 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:
  * 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 11 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef MediaStreamTrackSourcesCallback_h
 #define MediaStreamTrackSourcesCallback_h
 
 #include "modules/mediastream/SourceInfo.h"
 namespace blink {
 
-class MediaStreamTrackSourcesResponse;
-
 class MediaStreamTrackSourcesCallback : public GarbageCollectedFinalized<MediaStreamTrackSourcesCallback> {
 public:
     virtual ~MediaStreamTrackSourcesCallback() { }
     virtual void trace(Visitor*) { }
     virtual void handleEvent(const SourceInfoVector&) = 0;
 };
 
 } // namespace blink
 
 #endif // MediaStreamTrackSourcesCallback_h