Optimize getTopSites

[bbondy]

Fix #10004

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).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

• Re-test top sites completely (new tab tiles)
  • For example clear private data, make sure it clears the top sites back to default
  • There will be a slight delay (intentional) on re-calculating top sites, within 5 seconds after page load.

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]

ready for review

[bbondy]

There's the end result under the name debounce, the test was run over 20 page views. Before it was over 7% of the time.

[bsclifton]

Changes look and work great 😄 Awesome find 👍

BTW: @bbondy any manual QA needed? If so please add steps; if not, please add QA/no-qa-required tag. Thanks 😄