Move MediaStream and MediaStreamTrack implementation from modules/mediastream to core/mediastream.

Source/modules/EventTargetModules.h

 // Copyright (c) 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef EventTargetModules_h
 #define EventTargetModules_h
 
 #include "core/events/EventTarget.h"
 #include "modules/EventModulesNames.h"
 #include "modules/EventTargetModulesNames.h"
 
 namespace blink {
 
 class AudioContext;
 class FileWriter;
 class IDBDatabase;
 class IDBRequest;
 class IDBTransaction;
 class MIDIAccess;
 class MIDIInput;
 class MIDIPort;
-class MediaStream;
 class Notification;
 class ExecutionContext;
 class ScriptProcessorNode;
 class DOMWebSocket;
 
 } // namespace blink
 
 #endif // EventTargetModules_h