Validate HTML tags in UI strings (1.16.x).

[mkarolin]

Fixes brave/brave-browser#11767
Fixes brave/brave-browser#11972

Uplift from #6720
Uplift from #6761

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, npm run gn_check)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Requested a security/privacy review as needed.
• Added appropriate QA labels (QA/Yes or QA/No) to the associated issue
• Added appropriate release note labels (release-notes/include or release-notes/exclude) to the associated issue
• 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:

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.

[kjozwiak]

Restarting macOS via https://ci.brave.com/job/pr-brave-browser-maxk-check-html-tags-in-strings-1.16.x-macos/5/ using WIPE_WORKSPACE due to init failing as per https://ci.brave.com/job/pr-brave-browser-maxk-check-html-tags-in-strings-1.16.x-macos/4/execution/node/104/log/

15:26:46  Error: Command 'vpython src/third_party/depot_tools/download_from_google_storage.py --no_resume --directory --recursive --no_auth --num_threads=16 --bucket chromium-apache-mac src/third_party/apache-mac' returned non-zero exit status 1 in /Users/jenkins/jenkins/workspace/pr-brave-browser-maxk-check-html-tags-in-strings-1.16.x-macos
15:26:46  Hook 'vpython src/third_party/depot_tools/download_from_google_storage.py --no_resume --directory --recursive --no_auth --num_threads=16 --bucket chromium-apache-mac src/third_party/apache-mac' took 27.79 secs

[kjozwiak]

Uplift into 1.16.x approved after deliberating with @brave/uplift-approvers.