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This allows proper UI update (color change) on tabs that have been added
to a group.

Had to remove the unit test ensuring contiguity across all update
notifications, since it's no longer a valid assumption.

Bug: 976502
Change-Id: I260eb4fb5128079068f7750b882dcb1d749eceed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1670281
Commit-Queue: Dana Fried <dfried@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671489}