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Add a delete-button, implemented with a <cr-button>, in-between the
open-button and metadata-info-button.

Change tab-index tests to support the delete-button.

Bug: 803259
Change-Id: Ifee06c3acdb346c5b54b7a4895926cc2406f44c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2043334Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Commit-Queue: Rachel Sugrono <rsugrono@google.com>
Cr-Commit-Position: refs/heads/master@{#739308}