Allow view-source as openable by context menu

[bsclifton]

Fixes #15075

Auditors: @diracdeltas

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. Visit https://brave.com
2. Right click the page, pick View Page Source. Ensure page source comes up. Close it
3. Manually type a view source URL into URL bar: view-source:https://clifton.io. Ensure page source comes up. Close it
4. Make a new bookmark for view-source:https://brianbondy.com/ and launch the bookmark. Ensure page source comes up

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]

master 75b8cda
0.24.x 48841b1
0.23.x 18efca1