Replacing last tab closed with a new tab (Rather than closing browser window)

[ryanml]

Fixes #5873

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:

(Should be tested with multiple windows open to ensure containment)

1. Open a few tabs (Test with some pinned)
2. Close all but one
3. Upon closing the last tab, ensure that it is 'replaced' with about:newtab rather than closing the browser window
4. If multiple windows are open, ensure this only happens in the intended window and there are no misfires.

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

There seemed to be some ambivalence on the issue as to whether we should have a toggle for this behavior or enforce it as a standard. I'll be happy to add in this preference to preferences#tabs if we feel it should be there.

@bradleyrichter @srirambv let me know your thoughts, thanks!

[codecov-io]

Codecov Report

Merging #13577 into master will increase coverage by 0.05%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master   #13577      +/-   ##
==========================================
+ Coverage   56.71%   56.77%   +0.05%     
==========================================
  Files         285      285              
  Lines       28797    28784      -13     
  Branches     4755     4756       +1     
==========================================
+ Hits        16332    16341       +9     
+ Misses      12465    12443      -22

Flag	Coverage Δ
#unittest	56.77% <90.9%> (+0.05%)	⬆️

Impacted Files	Coverage Δ
app/browser/reducers/tabsReducer.js	41.47% <90.9%> (+1.47%)	⬆️
js/stores/windowStore.js	28.75% <0%> (+0.29%)	⬆️
app/renderer/components/reduxComponent.js	61.2% <0%> (+3.44%)	⬆️
js/stores/appStoreRenderer.js	100% <0%> (+8.33%)	⬆️

[petemill]

I believe this is something requested by @bradleyrichter cc @davidtemkin so maybe should be discussed and prioritized to an upcoming milestone.

[bsclifton]

Closing as the issue this fixes has been closed