add advanced webrtc IP handling preference

[diracdeltas]

fix #13668

Test Plan:

1. go to about:preferences#advanced
2. at the bottom, it should show a webrtc policy select menu which defaults
   to 'default'
3. turn on fingerprinting protection to 'block all'
4. go to https://browserleaks.com/webrtc. it should not show any IPs
5. turn off fingerprinting protection on that page. now it should show IPs
6. in about:preferences#advanced, set webrtc policy to 'default public
   interface only'
7. reload https://browserleaks.com/webrtc. it should only show the
   public IP.
8. in about:preferences#advanced, set webrtc policy to 'disable
   non-proxied UDP'
9. reload https://browserleaks.com/webrtc. it should show no IPs.
10. in about:preferences#advanced, set webrtc policy to 'default public
    and private interfaces'
11. reload https://browserleaks.com/webrtc. it should show both IPs.

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.)

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

[codecov-io]

Codecov Report

Merging #13713 into master will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master   #13713      +/-   ##
==========================================
- Coverage   56.53%   56.51%   -0.03%     
==========================================
  Files         282      283       +1     
  Lines       28747    28756       +9     
  Branches     4764     4765       +1     
==========================================
- Hits        16253    16251       -2     
- Misses      12494    12505      +11

Flag	Coverage Δ
#unittest	56.51% <66.66%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
js/constants/settings.js	100% <ø> (ø)	⬆️
js/constants/appConfig.js	100% <100%> (ø)	⬆️
js/constants/webrtcConstants.js	100% <100%> (ø)
app/renderer/components/preferences/advancedTab.js	48.43% <50%> (+0.89%)	⬆️
app/browser/reducers/tabsReducer.js	38.05% <62.5%> (-1.89%)	⬇️
js/state/siteSettings.js	92.71% <0%> (-0.67%)	⬇️

[bsclifton]

@diracdeltas @bradleyrichter does it make sense to have this new setting on the Advanced page? Or would it make more sense to be on the Security page? Doesn't seem like an intuitive place to find it

[diracdeltas]

@bsclifton i put it in advanced because probably 99.9% of users won't know what the options mean or care to change the default. it could go either way. it's more of a networking setting than a security setting.

[bsclifton]

Clicking the What do these policies mean links me to https://cs.chromium.org/chromium/src/content/public/common/webrtc_ip_handling_policy.h

Looking at the comments in the code, I'm able to map these constants (and the comments above them) to what we have in our drop-down... but I think it might be very difficult for users to do so (unless they're also programmers).

I think it's good for a rev1 though- just something to keep in mind. Can definitely be improved in iterations 😄

[diracdeltas]

@bsclifton noted. that's partly why i put it in advanced. :)

[bsclifton]

Changes work great per test plan 😄 Code changes look great and unit tests pass and accurately cover the change

[bsclifton]

master 0640d1e
0.23.x 290aeae
0.22.x efb6542