[WIP] fix: load remote images in untrusted iframe

[josheleonard]

Resolves https://github.com/brave/brave-browser/issues/24527

Submitter Checklist:

• [ ] I confirm that no security/privacy review is needed, or that I have requested one
• [x] There is a ticket for my issue
• [x] Used Github auto-closing keywords in the PR description above
• [x] Wrote a good PR/commit description
• [x] Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• [x] 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:

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[diracdeltas]

thanks, looks like my concerns have been addressed. will let @kdenhartog approve this on behalf of sec team.

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[josheleonard]

Closing. Loading numerous iFrames had a noticeable performance impact