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.

It would be something like:

1. Let header be the result of getting Clear-Site-Data from response's header list.
2. If header is non-null, then:
   1. Let headerTypes be the result of parsing header per the 1#( quoted-string ) ABNF.
   2. For each type of headerTypes, ...

Let me know if you have any concerns with this.