Don't prompt for restart when changing Toolbar and UI element scale (…

[bsclifton]

…since the underlying problem was fixed)

Fixes #10089

Auditors: @luixxiul, @alexwykoff

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.

Test Plan:

1. Go to Preferences > Advanced
2. Under "Toolbar and UI Elements scale", pick a different value from the drop down
3. you'll be prompted to restart. Ignore this or click "no"
4. Notice that the top area (where the URL bar is) has changed in size
5. Notice that you are no longer prompted to restart

Reviewer Checklist:

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 #10578 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10578      +/-   ##
==========================================
- Coverage    54.2%   54.19%   -0.02%     
==========================================
  Files         244      244              
  Lines       21106    21109       +3     
  Branches     3258     3258              
==========================================
- Hits        11441    11440       -1     
- Misses       9665     9669       +4

Flag	Coverage Δ
#unittest	54.19% <ø> (-0.02%)	⬇️

Impacted Files	Coverage Δ
js/about/preferences.js	43.38% <ø> (-0.48%)	⬇️

[luixxiul]

Tested locally and works great. LGTM.

[NejcZdovc]

++