Media Session API: make MediaMetadata mutable.

third_party/WebKit/Source/modules/mediasession/MediaMetadata.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://wicg.github.io/mediasession/#the-mediametadata-interface
 
 [
     ConstructorCallWith=ExecutionContext,
     Constructor(MediaMetadataInit metadata),
     RuntimeEnabled=MediaSession,
 ] interface MediaMetadata {
-    readonly attribute DOMString title;
-    readonly attribute DOMString artist;
-    readonly attribute DOMString album;
-    readonly attribute FrozenArray<MediaImage> artwork;
+    attribute DOMString title;
+    attribute DOMString artist;
+    attribute DOMString album;
+    attribute FrozenArray<MediaImage> artwork;
 };