Allow popups from inside brave://getting-started iframe

[bsclifton]

Fixes brave/brave-browser#43719

Security review https://github.com/brave/reviews/issues/1858

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:

1. Launch Brave
2. Visit brave://getting-started
3. Scroll down to very bottom and click button Learn more
4. Link should open in a new tab
5. Go back to brave://getting-started
6. Scroll down (mid-way) to See a better web button and click it
7. Link should open in a new tab

[brave-builds]

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

[bsclifton]

Built locally and confirmed this works great!

Thanks for the review @zenparsing and thanks for the security review @diracdeltas 😄

[brave-builds]

Released in v1.77.17

[kjozwiak]

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

Brave | 1.77.33 Chromium: 133.0.6943.54 (Official Build) nightly (64-bit)
-- | --
Revision | f3ef3d8f315fd70c52dacb37981f31a53a8f0b49
OS | Windows 11 Version 24H2 (Build 26100.2894)

Using the STR/Cases outlined via #27497 (comment) & brave/brave-browser#43719 (comment), ensured that both See a better Web & Learn more were opening new tabs as per the following:

• ensured that clicking on See a better Web was opening https://search.brave.com/search?q=Best%20cheap%20laptops&source=welcomePages&summary=1 in a new tab without issues
• ensured that clicking on Learn more was opening https://brave.com/whats-new/ in a new tab without issues

Recording.2025-02-11.172123.mp4