[api-minor] Convert getViewerPreferences to return data in a Map

[Snuffleupagus]

Compared to regular Objects there's a number of advantages to using Maps:

• They support proper iteration.
• They have a simple way to check for the existence of data.
• They have a simple/efficient way to check the number of elements.

If this functionality was added today, I cannot imagine that we'd choose an Object for this data.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.85%. Comparing base (cd03276) to head (67ed457).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/core/catalog.js	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21607   +/-   ##
=======================================
  Coverage   89.84%   89.85%           
=======================================
  Files         263      263           
  Lines       66758    66762    +4     
=======================================
+ Hits        59980    59989    +9     
+ Misses       6778     6773    -5     

Flag	Coverage Δ
browsertest	66.55% <0.00%> (+0.01%)	⬆️
fonttest	9.05% <ø> (ø)
integrationtest	69.27% <0.00%> (+0.01%)	⬆️
unittest	57.49% <33.33%> (+<0.01%)	⬆️
unittestcli	56.45% <33.33%> (+<0.01%)	⬆️

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[timvandermeij]

r=me, with passing browser tests. Thanks!

[timvandermeij]

/botio test

[moz-tools-bot]

From: Bot.io (Windows)

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Received

Command cmd_test from @timvandermeij received. Current queue size: 0

Live output at: http://54.193.163.58:8877/45e963120908ff8/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_test from @timvandermeij received. Current queue size: 0

Live output at: http://54.241.84.105:8877/3db4a81d5782a83/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Failed

Full output at http://54.241.84.105:8877/3db4a81d5782a83/output.txt

Total script time: 46.82 mins

• Unit tests: Passed
• Integration Tests: FAILED
• Regression tests: FAILED

  errors: 7

Image differences available at: http://54.241.84.105:8877/3db4a81d5782a83/reftest-analyzer.html#web=eq.log

[moz-tools-bot]

From: Bot.io (Windows)

---

Failed

Full output at http://54.193.163.58:8877/45e963120908ff8/output.txt

Total script time: 79.63 mins

• Unit tests: Passed
• Integration Tests: FAILED
• Regression tests: Passed