Tweak behavior of #brave-adblock-cookie-list-default flag

[antonok-edm]

Resolves brave/brave-browser#20126

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

Note: As of the time of writing, there is a filter issue preventing the space for the cookie banner from being collapsed when it appears at the top of the page. If that's still the case during testing, please consider the notice to be "not shown" so long as the text is not visible.

1. Visit https://digikey.com and confirm that a cookie notice is shown. It will appear either as a banner at the top, or as a popup along the bottom depending on window size.
2. Visit brave://flags and enable the #brave-adblock-cookie-list-default flag, then restart the browser.
3. Visit https://digikey.com and confirm that no cookie notice is shown.
4. Visit brave://adblock and confirm that the entry for Easylist-Cookie List - Filter Obtrusive Cookie Notices under Additional filters exists and is checked.
5. Return to brave://flags and set #brave-adblock-cookie-list-default to Disabled, and restart the browser.
6. Visit https://digikey.com and confirm that the cookie notice is shown again.
7. Visit brave://adblock and confirm that the entry for Easylist-Cookie List - Filter Obtrusive Cookie Notices is no longer checked.
8. Manually toggle the Easylist-Cookie List - Filter Obtrusive Cookie Notices entry to be enabled, then repeat steps 2-5.
9. Visit brave://adblock and confirm that the entry for Easylist-Cookie List - Filter Obtrusive Cookie Notices is checked.
10. Manually toggle the Easylist-Cookie List - Filter Obtrusive Cookie Notices entry to be disabled.
11. Visit brave://flags and enable the #brave-adblock-cookie-list-default flag, then restart the browser.
12. Visit brave://adblock and confirm that the entry for Easylist-Cookie List - Filter Obtrusive Cookie Notices is unchecked.
13. Visit https://digikey.com and confirm that the cookie notice is shown.

[emerick]

Seems like a unit test to exercise this new preference could be useful.

[antonok-edm]

This is meant to be a pretty simple temporary change so we can use Griffin to evaluate whether or not it's worth pulling the cookie list into a better more permanent position, but point taken, I know how these "temporary" features go 😅

I couldn't see an easy way to test this in a unit test, but I've added a browsertest.

[emerick]

LGTM

[antonok-edm]

Windows error unrelated, merging.