Remove required and [EnforceRange] attributes from readonly dictionaries

[twiss]

Remove the required and [EnforceRange] attributes of all properties of KeyAlgorithm dictionaries, which are only used on the algorithm property of CryptoKey, which is readonly.

WebIDL says:

Note that specifying dictionary members as required only has an observable effect when converting other representations of dictionaries (like a JavaScript value supplied as an argument to an operation) to an IDL dictionary. Specification authors should leave the members optional in all other cases, including when a dictionary type is used solely as the return type of operations.

and:

A type annotated with the [EnforceRange] extended attribute must not appear in a read only attribute.

(even though the attributes themselves are not readonly, the algorithm attribute of CryptoKey is).

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[annevk]

I have not verified that these dictionaries are only used as return values.

(I also think we should change Web IDL at some point so required is fine for return values as it's kinda useful there for code generation and indicating the member will be present.)

[twiss]

Right, sorry, I saw your other comment too late.

It's a bit out of scope here but maybe it would be sensible to flip the defaults, and have optional and [Modulo]/[Truncate] or something like that instead (as also suggested in #429 (comment))? That way, you can still assume that returned dictionaries behave "sensibly" by default, without having to annotate all of them. It might cause a lot of churn for dictionaries that do get passed in, though.

Edit: created whatwg/webidl#1628 and whatwg/webidl#1629 for this.