add setting for enabling site isolation

[diracdeltas]

fix #12490

Test Plan:

1. go to preferences > Security
2. enable Strict Site Isolation at the bottom of the page
3. restart
4. open devtools and go to https://jsfiddle.net/avse30aj/. you should not see any requests to fiddle.jshell.net in the network requests panel.

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:

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

[diracdeltas]

this seems to work in the sense that it creates a new process for the iframe. however, for some reason, the iframe crashes on https://jsfiddle.net/avse30aj/ with site isolation enabled in Brave whereas it loads fine in Chrome with site isolation enabled

[codecov-io]

Codecov Report

Merging #12491 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12491      +/-   ##
==========================================
- Coverage   55.91%    55.9%   -0.01%     
==========================================
  Files         278      278              
  Lines       26975    26978       +3     
  Branches     4363     4363              
==========================================
+ Hits        15082    15083       +1     
- Misses      11893    11895       +2

Flag	Coverage Δ
#unittest	55.9% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
js/constants/appConfig.js	100% <ø> (ø)	⬆️
js/constants/settings.js	100% <ø> (ø)	⬆️
js/about/preferences.js	45.73% <ø> (-0.08%)	⬇️

[diracdeltas]

opened follow-up issue: brave/muon#422

[diracdeltas]

master: aba789a
0.21.x: 3f672e4
0.20.x: 35454c9
0.19.x: e2d0ed6