NOT FOR LANDING Introduce and use WebMediaSession

Source/modules/mediasession/MediaSession.idl

Index: Source/modules/mediasession/MediaSession.idl
diff --git a/Source/modules/mediasession/MediaSession.idl b/Source/modules/mediasession/MediaSession.idl
index 235604b3f20f4b98c5470611e3d8e68f8152db2e..560fca24a8da61827c19ad6cdd0258d75f17fb7b 100644
--- a/Source/modules/mediasession/MediaSession.idl
+++ b/Source/modules/mediasession/MediaSession.idl
@@ -6,9 +6,10 @@
 
 [
     Constructor(),
+    ConstructorCallWith=ScriptState,
     GarbageCollected,
     RuntimeEnabled=MediaSession,
 ] interface MediaSession {
-    void activate();
-    void deactivate();
+    [CallWith=ScriptState] Promise<void> activate();
+    [CallWith=ScriptState] Promise<void> deactivate();
 };