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Without this attribute, screen readers don't announce whether a tab is
currently selected.

Bug: 1123470
Change-Id: Ie88959bf1cd4fc68003fffa62fbebe57877b899f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410496
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: dpapad <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807511}