don't override website's document.adoptedStyleSheets

[SergeyZhukovsky]

Resolves brave/brave-browser#14333

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:

[antonok-edm]

if this can overwrite a website's stylesheet, is it possible for websites to "clear" cosmetic filtering that we set this way too?

[SergeyZhukovsky]

Yes, it is

[SergeyZhukovsky]

perhaps a way to prevent that is to check and reapply when we do passes for 1st party to unhide

[kjozwiak]

Verification PASSED on Win 10 x64 using the following build:

• verified the STR/cases outlined via Brave overwrites document.adoptedStyleSheets brave-browser#14333 (comment) and ensured text color was being displayed correctly.

1.20.110 CR: 88.0.4324.192	1.23.10 CR: 89.0.4389.69

[srirambv]

Verification passed on OnePlus 6T with Android 10 running 1.23.9 x64 nightly build

• Verified text colour was blue as expected
• Verified changing cosmetic filter flag didn't affect it