Allow modification of window attributes with scriptlets

[antonok-edm]

Resolves brave/brave-browser#9496

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).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• 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.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. 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-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

To manually test, visit https://www.reuters.com/article/us-health-coronavirus-usa-election/its-not-about-you-democrats-bet-trump-coronavirus-response-a-2020-winner-for-biden-idUSKCN2292UN and verify that there is no "PAID PROMOTIONAL LINKS" section at the bottom of the page after a few seconds of load time.

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

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.

[pes10k]

lgtm!

[bsclifton]

• Tested and verified it works as expected (current Nightly versus the binaries spit out by PR builder)
• Code changes look good!
• CI looks good:
  • macOS had a test-install failure (known)
  • Android has failing unit tests (known and broken on master, not related to this; folks working on fixing)
  • Windows has a specific unit test failure that would be solved if this was rebased

Ready to merge 👍

[bbondy]

Better to just base64 encode the actual value for next time base::Base64Encode.