Rewrite CreateCookiePartitionKey to take a callback.

Bug: 1283271, 1266014
Change-Id: I858df81db39e4d5b0a504b82a58a939d602edb2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3387868


Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Chris Fredrickson <cfredric@chromium.org>
Cr-Commit-Position: refs/heads/main@{#962175}