Just to add more detail on the auth use case: What we need is to detect when a particular cookie has been set on a particular domain and then read its value. We know the domain at the time we create the web.Context.

On Android, CookieManager[1] provides this functionality and so may be a useful reference point for design. Note that also includes controls for manipulating cookies and for controlling the cookie policy, neither of which are required in the Auth use case. I'm not able to speak for other Fuchsia use cases.

1 - https://developer.android.com/reference/android/webkit/CookieManager 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41

commit acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41
Author: Kevin Marshall <kmarshall@chromium.org>
Date: Wed Sep 26 21:05:47 2018

Fuchsia: Integrate CookieStore with Context service.

This CL adds cookie support to the browser, so that the content layer
can set and query cookies.

* TestLauncher modified to give isolated /data directories to child
  processes.
* Browsertests added to test cookie support in incognito/in-memory and
  persisted cases.


Bug: 858853
Change-Id: Ia22e27778323e5b08fd872a2b29b4c7d879f80a0
Reviewed-on: https://chromium-review.googlesource.com/1168479
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594470}
[modify] https://crrev.com/acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41/base/test/launcher/test_launcher.cc
[modify] https://crrev.com/acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41/webrunner/browser/context_impl.h
[modify] https://crrev.com/acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41/webrunner/browser/context_impl_browsertest.cc
[modify] https://crrev.com/acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41/webrunner/browser/webrunner_browser_context.cc
[modify] https://crrev.com/acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41/webrunner/browser/webrunner_browser_test.cc
[modify] https://crrev.com/acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41/webrunner/browser/webrunner_browser_test.h
[modify] https://crrev.com/acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41/webrunner/browser/webrunner_url_request_context_getter.cc
[modify] https://crrev.com/acb8e3bdf1e30d72450d15fbd02df5ad10b8fb41/webrunner/browser/webrunner_url_request_context_getter.h