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This change exposes hierarchical level for tablist in the accessibility
tree. Hierarchical level is determined from `aria-level` or the DOM
structure of the tablist in relation to its parent tablist.

https://www.w3.org/TR/wai-aria-1.1/#tablist
https://www.w3.org/TR/wai-aria-1.1/#aria-level

Change-Id: I1d6aba1bf47b3998a7622a98ec75dae1e41c9452
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1996263
Commit-Queue: Victor Fei <vicfei@microsoft.com>
Reviewed-by: Ian Prest <iapres@microsoft.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738049}