Disable WebTorrent extension by default

[bsclifton]

Disables WebTorrent for NEW users only by default.

Existing users will keep their current value. This code will "lock the value in" when it runs the first time.

Fixes brave/brave-browser#44303

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• 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 wiki
  • npm run presubmit wiki, 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/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

New user

1. Blank profile
2. Launch Brave
3. Visit brave://settings/extensions
4. Ensure WebTorrent is set to FALSE

Existing user - explicitly enabled

1. Have a profile BEFORE this change
2. Toggle the value for WebTorrent on brave://settings/extensions
   • Disable it
   • Enable it
3. Launch a version of Brave which has this update
4. Visit brave://settings/extensions
5. Verify WebTorrent is set to TRUE

Existing user - they never touched it

1. Have a profile BEFORE this change
2. Launch a version of Brave which has this update
3. Visit brave://settings/extensions
4. Verify WebTorrent is set to TRUE

Existing user - making a new profile

1. Do steps for Existing user - they never touched it
2. Use Hamburger menu > More tools > Add new profile to create a new profile
3. Visit brave://settings/extensions
4. Verify WebTorrent is set to FALSE

[simonhong]

++ with nits!