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We've had several bugs where tab groups could become discontinuous and
it would be nice if we didn't crash when this happens, so the user can
recover. This patch adds a tab reference count to groups, so they are
guaranteed to not have any tabs in them when they are deleted, even if
the group is not continuous.

Bug: 905491
Change-Id: If585e0c5169830280efa09b37402dbd3367359e1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1709160
Commit-Queue: Charlene Yan <cyan@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679566}