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- Tabs will read out their group membership.
- Group headers will read out their tab contents.

Tiny side bonus: Focusing on group headers will no longer show the wrong hovercard.

Bug: 1045844
Change-Id: I5fc98231a5c5a0372aa75e31a5c3a6e321ce24b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2025698
Commit-Queue: Connie Wan <connily@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736582}