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This is allowing the user to use Tab and up/down keys to navigation
throughout the menu. It can also be closed using the Escape key.

This change also trap the focus inside the menu while it's visible and
make it so that hovenig an item will move the focus to it, like most
menus will do.

This is mostly for accessibility so users can interact with the
controls using only a keyboard. The change applies to both the overflow
menu and the text track list.

Bug: 821131
Change-Id: I22db341d0d55a8e3297b3c7a18c92331e296cd9e
Reviewed-on: https://chromium-review.googlesource.com/1013478Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551353}