Load search favicons locally in about:preferences

[diracdeltas]

fix #12689
fix #11108
partial fix for #12632

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. go to about:preferences and open the 'network requests' tab in devtools
2. click on the search preferences tab. you should not see any network requests to remote sites.
3. all the search favicons should load correctly

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

[tildelowengrimm]

This looks like the exact MVP for #12689. 👌

Is the ico/png variation just because that's what these sites are serving today? I'm r+, but I haven't checked that the ico files are correct (or that any of the images aren't malicious blobs).

I think that there are two bigger blocks of subsequent work to polish this feature. One is safely automating our inclusion of updated favicons when sites occasionally change them — so we don't need to watch for changes or update the codebase every time. The other one is updating the favicons based on user browsing, but that seems like a fair bit more work because we'd want to depend on a favicon cache. I'll open other issues for those.

[codecov-io]

Codecov Report

Merging #12698 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #12698      +/-   ##
==========================================
+ Coverage   56.18%   56.19%   +<.01%     
==========================================
  Files         278      278              
  Lines       27121    27127       +6     
  Branches     4395     4396       +1     
==========================================
+ Hits        15239    15245       +6     
  Misses      11882    11882

Flag	Coverage Δ
#unittest	56.19% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
js/data/searchProviders.js	100% <100%> (ø)	⬆️
js/about/preferences.js	45.73% <100%> (ø)	⬆️

[diracdeltas]

Is the ico/png variation just because that's what these sites are serving today? I'm r+, but I haven't checked that the ico files are correct (or that any of the images aren't malicious blobs).

Yup! I just ran wget on the URL in the image property (which were all HTTPS) in order to get the localImage file.

[diracdeltas]

master: 574e319
0.21.x: 2f9a6ca
0.20.x: 54ad151