modify title case for win and linux on menu

[SebastianLF]

Resolves this issue: brave/brave-browser#17377
Modify New Private window to New private window on menu for Windows and Linux.

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

[mkarolin]

Sadly, this will not work. _override.grd files are auto-generated by running npm run rebase_chromium_l10n. The strings in the _override files are based on automatic generic substitutions that are mapped in https://github.com/brave/brave-core/blob/master/script/lib/grd_string_replacements.py
There is no mapping for Incognito -> private (maybe it's safe to add one if we adjust regex to make sure it's not the first word in the phrase? cc: @bsclifton). Barring that, I think the best option for this is to add a new string into app/brave_generated_resources.grd and then replace IDS_NEW_INCOGNITO_WINDOW with our string ID in code via a chromium_src override.

[bsclifton]

Thanks for clarification @mkarolin 😄 I updated https://github.com/brave/brave-browser/wiki/Strings-and-Localization#chromium-strings to capture that detail (I had forgotten myself where the _override.grd comes from - just remembering I had asked here a while back brave/brave-browser#17377 (comment))

I like the suggestion of using a chromium_src override (versus trying to hack regex). This unfortunately takes it out of the good first issue category

[SebastianLF]

I'll let you take it from here :)

[bsclifton]

Thanks again for the attempt @SebastianLF - apologies for the complexity we ran into! But at least I was able to update the issue with more details and also I was able to update wiki capturing how the _override.grd files are created 😄