Add MediaSession runtime flag along with stub MediaSession object

Source/modules/mediasession/MediaSession.h

+// Copyright 2015 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 MediaSession_h
+#define MediaSession_h
+
+#include "bindings/core/v8/ScriptWrappable.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class MediaSession : public ScriptWrappable, public RefCountedWillBeNoBase<MediaSession> {
+    DEFINE_WRAPPERTYPEINFO();
+public:
+    static RefPtrWillBeRawPtr<MediaSession> create();
+
+    void activate();
+    void deactivate();
+
+private:
+    MediaSession();
+};
+
+}
+
+#endif // MediaSession_h