Only handle torrent URLs when WebTorrent extension is enabled

[yrliou]

Fix brave/brave-browser#2043
See also brave/brave-browser#2045 for adding disable_webtorrent_extension into npm start options.
This PR add checks in the backend so we will only redirect/rewrite magnet/torrent URLs when webtorrent extension is enabled.
A command line switch is also added to disable webtorrent extension.

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) on
  • Windows
  • macOS
  • Linux
• 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

Test Plan:

manual test:

1. Open the app from command line and pass --disable-webtorrent-extension
2. Visit https://webtorrent.io/free-torrents, click a torrent link will prompt to download the file
3. Click a magnet link will prompt to open it using your default application (ex: uTorrent)
4. Right click a magnet link to open it in a new tab, it will prompt to open it using your default application in the new tab.

npm test brave_browser_tests -- --filter=BraveContentBrowserClientTest.*

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

[yrliou]

master: 522accc
0.58.x: d4ae08e