Add a flag to mark a tab group is pending going private

This CL adds a flag to mark a tab group is going private,
this flag will be persisted to the database to avoid Chrome
crash before receiving server response. Here is the flow:
1. when a group is unshared, the flag is changed to true and
saved to db. If Chrome crashes, the flag can be loaded to
indicate that unshare has started.
2. Followup CL: when later the server tells Chrome the group
unsharing is finished, the flag is checked and a clone of the
group will be created and saved locally.

Low-Coverage-Reason: TESTS_IN_SEPARATE_CL
Bug: 383163590
Change-Id: I6f1db175453e8e3e914abb0686762234b527843f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6081325
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: David Pennington <dpenning@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1394969}