Add duckduckgo.com to whitelist for Brave UA

[bbondy]

Fix #10020

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).
  • It's just modifying a data file so I don't think anything is needed. onBeforeSendHeaders could benefit from a test but it's not newly introduced here.
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:
See #10020 (comment)

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bsclifton]

++