Address feedback in PR #8894

[ayumi]

#8894

Auditors: @bbondy

Test Plan:
Check that automated tests pass.

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).
• Ran git rebase -i to squash commits (if needed).

Test Plan:

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

[bbondy]

Did you check if this gets called frequently?

[ayumi]

I did check. It gets called once per newtab tab square, but it runs in <= 1ms.

[bbondy]

++