FF129 MediaCapabilities.decodingInfo() - encrypted content

[hamishwillee]

FF129 Adds support for using MediaCapabilities.decodingInfo() with encoded media in https://bugzilla.mozilla.org/show_bug.cgi?id=1898344

The spec is a bit messy, but what it means is that

• the configuration object passed to the decodingInfo() method has a new property called keySystemConfiguration.
• The returned promise resolves to include the property keySystemAccess which is a MediaKeySystemAccess you can use to get keys and decode the content for playback.

What I have done is added a subfeature under decodingInfo() for the keySystemConfiguration parameter. I haven't added another subfeature for the keySystemAccess in the returned object - should I add that too, even though it would be identical?

Or is to have just one entry but name it something like encrypted_media?

Related docs work can be tracked in mdn/content#34696

Note: The spec has another value configuration in the return object, but that is not implemented by anyone.

[hamishwillee]

@Elchi3 Could I please have a review on this?

[Elchi3]

LGTM, thanks Hamish! 👍