Match Chromium behavior for extension loading

[bsclifton]

Fixes brave/brave-browser#15024

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• 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:

Verify you CAN NOT install on macOS or Windows

1. New profile (macOS or Windows); launch Brave
2. Download chromium-cleaner
3. Visit brave://extensions
4. Drag the chromium-cleaner.crx extension from desktop onto the chrome://extensions page
5. Verify extension shows in brave://extensions as disabled (shows warning; you can't enable). Should look like this:
   
6. Enable Developer Mode in top right of extensions page
7. Try steps 4 + 5 again; should fail again

Verify you CAN install on Linux

1. New profile (Linux); launch Brave
2. 2. Download chromium-cleaner
3. Visit brave://extensions
4. Drag the chromium-cleaner.crx extension from desktop onto the chrome://extensions page
5. Verify extension shows in brave://extensions as disabled (shows warning; you can't enable). Should look like this:
   
6. Enable Developer Mode in top right of extensions page
7. Try steps 4 + 5 again; this time it should work

[bsclifton]

Whoops - for Android, I need to fix GN dependency for test/BUILD.gn. Android build is trying to add the dep which doesn't work, resulting in:

13:44:00  ERROR at //chrome/browser/extensions/BUILD.gn:17:1: Assertion failed.
13:44:00  assert(enable_extensions)
13:44:00  ^-----
13:44:00  See //testing/test.gni:155:25: which caused the file to be included.
13:44:00                deps += [ _dep ]
13:44:00                          ^---

[bsclifton]

@brave/patch-reviewers @fmarier can I get a review on this please? 😄 Test plan added!

[bsclifton]

Ready for re-review, @mkarolin 😄

[mkarolin]

chromium_src changes LGTM, leaving security implications for @fmarier

[fmarier]

I've not looked too deeply into the implementation of these checks in Chromium, but +1 on matching the Chrome behavior.

[bsclifton]

OK great - CI looks good; there was one lint error with a test that I'll fix and commit. Ready to merge 😄