Add feature flag to toggle dark mode blocking when fingerprinting=strict

[ShivanKaul]

Resolves brave/brave-browser#17139.

I'm working on a fix to disable Brave Shields when there's a navigation error which will address general issues of that class (plus remove confusing UX), but in the meantime, adding an opt-out for folks who don't want fingerprinting strict to mess with their dark mode.

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
• 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:

[goodov]

remove k from the string: "kBraveDarkModeBlock" -> "BraveDarkModeBlock"

[ShivanKaul]

done

[IcyMidnight]

Thanks for adding this flag! It would be really nice to get it promoted to a configuration option in the main privacy settings view. Any appetite for that?

[ShivanKaul]

We considered that, but we want to avoid cluttering the privacy settings (there's already a lot there). Happy to take a look at / help with a PR though.

[meduzen]

For anyone coming here via search engines, the follow-up seems to be on brave/brave-browser#21420.