Fix broken PublicKeyCredentialJSON WebIDL definition

[MasterKale]

Update the WebIDL definition of the shape of PublicKeyCredentialJSON based on feedback in #1958.

NOTE: I have no idea how to validate that this is actually going to fix the issue.

Fixes #1958.

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[MasterKale]

I've added as reviewers individuals representing browser maintainers. I'd love to hear whether your IDL parsers can understand the WebIDL as-is, or not.

At the WG meeting today (9/27) I've heard the following about the ability to parse the current definition of PublicKeyCredentialJSON:

• @agl said Chrome's IDL parser could handle it
• @jschanck said Mozilla's IDL parser could not handle it

This PR is probably still relevant given we already have one project that can't parse it, which probably means the current definition is indeed invalid.

In addition, one that's unclear to me is if a comment is sufficient to communicate and constrain the desired shape of PublicKeyCredentialJSON that browsers take in and return from these methods.

[jschanck]

This is equivalent to what we implemented in Firefox. I don't have a strong feeling about the comment---IMO the text in Section 5.1 is already sufficient.

[pascoej]

WebKit's IDL parser is able to handle the current definition of PublicKeyCredentialJSON.

[agl]

Not thrilled, but it seems that the original was technically incorrect so we'll try this. Hopefully Typescript etc don't have too much of a problem.