Clarify the `Clone configuration` algorithm

[Paul Adenot]

When looking a Clone Configuration, and after having found a bug in Gecko, a DOM developer said:

This looks very underspecified in the spec. For one, I don't think any spec defines what assign a copy of config[m] to clone[m]
means. And either that can throw exceptions, and it should be specified what needs to happen, or it doesn't and we need to
figure out what to do in our implementation.

It can throw an exception, a minimal test case (adapted from our bug) would be:

var buf = new ArrayBuffer(4);
buf.transferToFixedLength()
VideoDecoder.isConfigSupported({"codec": "፝a", "description":buf})

Chromium says:

Uncaught (in promise) TypeError: Failed to execute 'isConfigSupported' on 'VideoDecoder': description is detached.

which makes sense to me, but we need to spec it.

[Paul Adenot]

Meaning checking if an exception has been thrown, and rejecting the promise.

[Paul Adenot]

Same deal with VideoDecoder.configure() with a detached array buffer. Curiously the configuration isn't cloned in this case, but the same behaviour in Chrome (which I also find sensible).

[Dan Sanders]

Conceptually this could also be part of the valid VideoDecoderConfig algorithm, which makes sense for something throwing a TypeError.

[Eugene Zemtsov]

I agree that it does sound like a part of config validation