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Specifically, test that:
- Focus is forwarded to the thumbnail of the active page when focusing
  on the thumbnail bar.
- Pressing the up/down arrows focuses on the correct thumbnails.
- Pressing the left/right arrows changes the page.

Bug: 1133840
Change-Id: Id689db821cff3c33e9d3ecee103532c5a3ee4b95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485564
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819467}