Redesign of "extract header list values" expected

[annevk]

Hi, I'm planning to change something you use, in whatwg/fetch#814. The new design should be relatively straightforward for you, provided implementations actually use the ABNF for their parsers (and I think there's tests for that for Referrer-Policy).

It would be something like:

1. Let policyValues be the result of getting Referrer-Policy from response's header list.
2. If policyValues is non-null, then:
   1. Let policyTokens be the result of parsing policyValues per the 1#(policy-token / extension-token) ABNF.
   2. For each token of policyTokens, ...

Let me know if you have any concerns with this.

[mikewest]

SGTM.

[annevk]

I think this could use https://fetch.spec.whatwg.org/#concept-header-list-get-decode-split, but I'm not sure what parsers are currently in use for this header.