Introduce request infrastructure for storage access

[cfredric]

This splits out the underlying infrastructure changes introduced in #213 so that #213 can just introduce the behavior change, and this PR can add the infrastructure needed to formally define the current behavior (in Chrome and Firefox; Safari TBD).

I also incorporated @annevk's request that request/eligible for storage-access be a tri-state enum, rather than a boolean.

This PR relies on the environment/ancestry enum, similar to other cookie layering work (e.g.).

• At least two implementers are interested (and none opposed):
  • Chrome
  • …
• Tests are written and can be reviewed and commented upon at:
  • https://crrev.com/c/6330426
• Implementation bugs are filed:
  • Chromium: https://crbug.com/379030052
  • Gecko: …
  • WebKit: …

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