fix tor-related webtorrent issues

[diracdeltas]

• re-enable webtorrent in private tabs
• stop page loading indicator after torrent warning is shown in a tor
  tab

fix #14472
fix #14524

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).
• 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. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

1. open brave and make sure webtorrent is enabled
2. go to https://webtorrent.io/torrents/sintel.torrent in a private tab.
   it should load webtorrent.
3. go to https://webtorrent.io/torrents/sintel.torrent in a tor tab. it
   should show a privacy warning.
4. click 'ok'. the page loading indicator should stop.

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

Test plan works great 😄👍