Implement ArrayBuffer overload of TextDecoder::decode.

[Ms2ger]

Blocks #5601.

• Spec: https://encoding.spec.whatwg.org/#dom-textdecoder-decode.
• Code: components/script/dom/textdecoder.rs.
• Test: /encoding/api-basics.html.

[yodalee]

I'm trying to implement this.
Do I need to modify webidls/TextDecoder.webidl and remove this line:

USVString decode(optional object input);

and uncomment this line, and add dictionary TextDecodeOptions

USVString decode(optional BufferSource input, optional TextDecodeOptions options);

Since the former is not in the interface spec: https://encoding.spec.whatwg.org/#interface-textdecoder

[jdm]

BufferSource is defined in the WebIDL spec, but our codegen can't handle the the ArrayBufferView or ArrayBuffer types yet. The first argument will need to stay as optional object input until we do.

[nox]

We have codegen for ArrayBuffer now, right?

[jdm]

No codegen support yet.

[jdm]

Duplicate of #20344.