Don't need to have rounded contents margin in tab-fullscreen

[simonhong]

fix brave/brave-browser#43136

Resolves

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• 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 wiki
  • npm run presubmit wiki, 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/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

See the linked issue.

[brave-builds]

Released in v1.76.1

[kjozwiak]

Verification PASSED on Win 11 x64 using the following build(s):

Brave | 1.76.1 Chromium: 132.0.6834.57 (Official Build) nightly (64-bit)
-- | --
Revision | 3140cfb88368b97c98ca36647dd22dc123ff2430
OS | Windows 11 Version 24H2 (Build 26100.2605)
JavaScript | V8 13.2.152.23

I've personally had this issue for ~2-3 months where thin white borders were appearing at the side of the monitor whenever watching any type of media in full screen. Updated my personal profile and ensured the browser border has been removed when watching YT or Netflix. Asked @okidokiokami to check/verify as well.

Broken	Fixed