Let MediaMetadata#artwork return the same value (unless set to a new value)

[foolip]

https://wicg.github.io/mediasession/#dom-mediametadata-artwork

The definition of the getter is to return a new array on every access. While Web IDL doesn't disallow this, avoiding this is the reason that sequence<T> is no longer allowed as an attribute type.

Rather than returning a new array each time, the setter could set an internal slot that is used in the getter. This is almost how it's implemented in Blink, except that a new frozen array is created.

[youennf]

I am not sure what we should do here. Implementations probably follow the spec
We anyway have an issue here since setting the attribute will not store the same object.
Maybe we should add methods instead but this seems late here.

[youennf]

Fixed by #343