NOT FOR LANDING Introduce and use WebMediaSession

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(),
+    ConstructorCallWith=ScriptState,
     GarbageCollected,
     RuntimeEnabled=MediaSession,
 ] interface MediaSession {
-    void activate();
-    void deactivate();
+    [CallWith=ScriptState] Promise<void> activate();
+    [CallWith=ScriptState] Promise<void> deactivate();
 };