context menu fix for cosmetic filter

[Snuupy]

Now ready for review.

Closes brave/brave-browser#4417

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
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests && npm run test-security) on
  • Windows
  • macOS
  • Linux
• 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-Protection-Mode
  • 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

Test Plan:

1. Visit http://raymondhill.net/ublock/adbox.html
2. Use right click => Block element via selector on red area of text (where it says Should be hidden)
3. Prompt should show up with a populated selector
4. Cancel prompt, go to https://cnn.com
5. Visit https://benzworld.org/forums/w126-s-se-sec-sel-sd/
6. Use right click => Block element via selector on picture of car on the top
7. Prompt should show mentioning how selector couldn't be found
8. Enter a custom selector (ex: table#header)
9. Confirm content is hidden

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.

[bsclifton]

Manually tested - works great 😄 Going to check out CI real quick here...

[bsclifton]

Verified CI looks good (I don't believe any of the failures are tied to this PR; ex: upload to s3). Good to go!