Cancel the margin-left of browserButton with groupedItem

[luixxiul]

Closes #11288

Auditors: @cezaraugusto

Test Plan:

1. Open about:styles
2. Click 'buttons'
3. Make sure the first button in a group does not have the left margin

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:

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 #11290 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11290      +/-   ##
==========================================
- Coverage   52.48%   52.45%   -0.04%     
==========================================
  Files         268      268              
  Lines       25229    25229              
  Branches     4026     4026              
==========================================
- Hits        13242    13233       -9     
- Misses      11987    11996       +9

Flag	Coverage Δ
#unittest	52.45% <ø> (-0.04%)	⬇️

Impacted Files	Coverage Δ
app/renderer/components/common/browserButton.js	69.76% <ø> (ø)	⬆️
js/stores/appStoreRenderer.js	91.17% <0%> (-8.83%)	⬇️
app/renderer/components/reduxComponent.js	84.37% <0%> (-6.25%)	⬇️
js/stores/windowStore.js	27.27% <0%> (-0.31%)	⬇️

[cezaraugusto]

++

[luixxiul]

As a follow up I'll add a comment to about:styles#buttons about https://github.com/brave/browser-laptop/pull/11290/files#diff-0ff973318abdc5c07f55cefe86bd5ed7R221 (The buttons have to have a parent. Beginning with Selectors Level 4, that is no longer required, adding a link to the MDN knowledge base's article which I referred)

#11522

[luixxiul]

@cezaraugusto would you mind adding this to 0.21.x? cc @bbondy

[cezaraugusto]

0.21.x 9e730c5

[bbondy]

confirmed it is already there:
0.21.x: dc237ec
master: 9e730c5

[luixxiul]

thanks!