don't override website's document.adoptedStyleSheets (uplift to 1.21.x)

[brave-builds]

Uplift of #8081
Resolves brave/brave-browser#14333

Pre-approval checklist:

• You have tested your change on Nightly.
• This contains text which needs to be translated.
  • There are more than 7 days before the release.
  • I've notified folks in #l10n on Slack that translations are needed.
• The PR milestones match the branch they are landing to.

Pre-merge checklist:

• You have checked CI and the builds, lint, and tests all pass or are not related to your PR.

Post-merge checklist:

• The associated issue milestone is set to the smallest version that the changes is landed on.

[kjozwiak]

audit-deps failed on post-init due to a known issue as per https://ci.brave.com/job/pr-brave-browser-pr8081_adoptedstylesheets_1.21.x-post-init/1/execution/node/144/log/.

test-browser failed on macOS due to BraveNavigatorUserAgentFarblingBrowserTest.FarbleNavigatorUserAgent failing as per https://ci.brave.com/job/pr-brave-browser-pr8081_adoptedstylesheets_1.21.x-macos/1/execution/node/241/log/ & https://ci.brave.com/job/pr-brave-browser-pr8081_adoptedstylesheets_1.21.x-macos/1/execution/node/244/log/ which is a known intermittent issue as per brave/brave-browser#13948.

[kjozwiak]

Uplift into 1.21.x approved after deliberating with @brave/uplift-approvers. QA has verified the PR on Nightly as per #8081 (comment) & #8081 (comment).