WIP: Redirect .onion URLs to a private window with Tor, if possible.

[riastradh-brave]

fix brave/brave-browser#806

• If there is no private window with Tor open, fail.

  => TODO: This should fail noisily, beyond a console message.

  => One may be tempted to just create a private window with Tor, but:
  . This is not obviously easy to do here.
  . Users may not be happy if simply clicking a link can have the
  effect of using Tor when they didn't intend.

• If there is a private window with Tor open, create a new tab to load
  the page.

  => User expectations around onion services are likely to be that
  they provide anonymity, so opening them in the non-anonymous
  window may be surprising.

  => It is not clear how easy it would be to load in a non-Tor window
  and ensure that everything is nevertheless loaded through Tor --
  and likewise links from it, &c.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests && npm run test-security) on
  • Windows
  • macOS
  • Linux
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. 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-Protection-Mode
  • 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-issues-with-tracking-protection

Test Plan:

1. Start a fresh Brave instance.
2. In a non-Tor window, open a .onion site like http://duskgytldkxiuqc6.onion/ or https://facebookcorewwwi.onion/
3. Confirm that it fails.
4. Open a private window with Tor.
5. Again in a non-Tor window, open the .onion site again.
6. Confirm that it opens in the private window with Tor instead.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

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.

[bbondy]

Is this still valid? No updates for a month. Thanks.

[riastradh-brave]

This is a draft that will likely need to be updated when we have finished reworking Tor profiles.

[bsclifton]

Closing as stale. We can keep the branch of course, in case we wanted to revisit