Updating'aria-selected' attributes for tabs on chrome://flags page

When tab is clicked it's 'aria-selected' attribute should be set to true,
and 'false' for other tabs.

Bug: 983373
Change-Id: Iee49632cd3b517e5c8f8bbe383ea2d7c59a44f98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1699545
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Commit-Queue: Andrii Sagaidak <ansagaid@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#678951}