Add stub for assigning media session to media elements

Source/modules/mediasession/MediaSession.idl

 // 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.
 
+// https://mediasession.spec.whatwg.org/#the-mediasession-interface
+
 [
     Constructor(),
     GarbageCollected,
     RuntimeEnabled=MediaSession,
 ] interface MediaSession {
     void activate();
     void deactivate();
 };