Make "is a registrable domain suffix of or is equal to" return the parsed domain

[masnoble]

What is the issue with the HTML Standard?

In a recent cookie store PR, we called into the "is a registrable domain suffix of or is equal to". Although this check internally runs the host parser for comparison, it unfortunately discards the parsed result, returning only a boolean (true or false). This necessitated re-running the host parser to store the parsed string, which is inefficient.

Ideally, the check should return the parsed string directly, allowing callers to utilize the already computed result.

CC: @annevk

[domenic]

We don't generally worry about efficiency in specs, so I think it's fine to not optimize this. See https://infra.spec.whatwg.org/#algorithm-conformance:~:text=Conformance%20requirements%20phrased%20as%20algorithms%20or%20specific%20steps%20may%20be%20implemented%20in%20any%20manner%2C%20so%20long%20as%20the%20end%20result%20is%20equivalent.%20(In%20particular%2C%20the%20algorithms%20are%20intended%20to%20be%20easy%20to%20follow%2C%20and%20not%20intended%20to%20be%20performant.)

[annevk]

I'd like to investigate if all callers are repeating themselves since I think it could be clearer if they would not have to.