Don't call unique() in content scripts when right clicking

[bsclifton]

Don't call unique() in content scripts when right clicking

unique call (to get selector for target) can be expensive; now only called when user chooses Block element via selector

Fixes brave/brave-browser#2410

Can likely be uplifted to 0.62.x since this fixes a perf issue

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

Test Plan:

1. Navigate to https://clifton.io/
2. Right click around the screen; verify context menu shows immediately
3. Right click something on the page and choose Brave => Block element via selector
4. Verify selector is shown in the modal
5. Navigate to https://mobile.twitter.com/welcomet0nature/status/1108858756018909185
6. Right click around the screen; verify context menu shows immediately
7. Right click something on the page and choose Brave => Block element via selector
8. Wait a REALLY long time
9. If the "Page is unresponsive. Continue waiting?" prompt shows, click "Wait"
10. Finally (maybe a minute later on debug builds), verify the selector is shown in the modal

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

[reaktivo]

@bsclifton This looks like a better solution. Hope this one get's merged soon.

[bsclifton]

Went through pipeline steps in Jenkins and verified the only step that failed was the security check (failing on js-yaml). Build on all platforms worked fine, tests passed. Security fix was handled by @jumde with brave/crypto#21