Since the entries in the MediaMetadata's `artwork` are frozen in the
current spec [1], the type of the attribute `artwork` must be
`FrozenArray<object>` rather than `FrozenArray<MediaImage>`. Otherwise
the entries of artwork can not be frozen [2]. This change will address
issue w3c#237

The `artwork` in `MediaMetadataInit` and `MediaMetadata` will be clearly
different after changing the `artwork` in `MediaMetadata` to
`FrozenArray<object>`, hence the _getter_, _setter_ of `artwork` and the
_convert artwork algorithm_ should be updated to match the change. This
change will address issue w3c#176

[1] https://github.com/web-platform-tests/wpt/blob/801a2b3b5e1cd0192f31890ddf9ee7b4d0ad9e89/mediasession/mediametadata.html#L148
[2] https://tc39.es/ecma262/#sec-object.freeze