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Allow keyboard focusing on thumbnails, and implement focused thumbnail
styling. The thumbnail bar is "focusable", but it forwards focus to the
thumbnail of the active page.

Focus can be changed among thumbnails using arrow keys. Tabbing while
focused on a thumbnail diverts focus out of the thumbnail bar.

Allow changing page of the PDF content using the enter and space keys.

Bug: 652400
Change-Id: Ie03a069d9feff2889723012810324803afad18b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2434770
Commit-Queue: Lei Zhang <thestig@chromium.org>
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811999}