Disable CNAME uncloaking when a proxy extension with a socks fallback is enabled

[antonok-edm]

Resolves brave/brave-browser#19070

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:

1. Install the Proxy SwitchyOmega extension
2. In the extension settings page, set the Proxy Servers section to have a single entry, as follows:

   Scheme	Protocol	Server	Port
   (default)	SOCKS5	127.0.0.1	8081

3. Clear the "Bypass List" section in the extension settings page
4. Ensure the above changes are applied by clicking the "Apply changes" button in the left column
5. Start a SOCKS5 proxy server on the device. This can be done by entering ssh 127.0.0.1 -D 8081 on Linux and probably macOS. You're welcome to run it on a different device, but the Proxy Servers setting above will need to be modified accordingly.
6. Add the following line to the Custom filters section in brave://adblock:

   ||dev-pages.brave.software/static/images/test.jpg
   

7. Visit https://test-cname.brave.software/cname-uncloaking.html
8. Use the Proxy SwitchyOmega icon in the "puzzle piece" extensions menu to enable proxy mode (should be the 3rd option).
9. Press the Run test button
10. The request should be allowed (green).
11. In the "puzzle piece" extensions menu, change to [System Proxy] and run the test again. The request should be blocked (red).
12. In the "puzzle piece" extensions menu, change to [Direct] and run the test again. The request should be blocked (red).

[fmarier]

Based on the Slack discussion, that looks good to me.

[darkdh]

We need unit test for ProxySettingsAllowUncloaking
but since we need to get this out ASAP, it can be done as follow up

[iefremov]

need to update the function comment, it describes only SingleProxy aka PROXY_LIST, not PROXY_LIST_PER_SCHEME

[antonok-edm]

Good call, updated the comment

[iefremov]

agree that we need a follow-up with tests