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If the only tab in a group is added to another group in a way that
causes that tab to move (e.g. there is an intervening ungrouped tab)
then we will crash. This is caused by TabStripModel setting the new
group on WebContentsData and then sending the kMoved notification before
the kGroupChanged notification. TabStripLayoutHelper::MoveTab expects
the group of the tab being moved is not empty according to the model.

This patch fixes the bug by sending kGroupChanged immediately after
setting the group on WebContentsData, so TabStrip will delete the old
group immediately.

Bug: 983961
Change-Id: Ib214a1b7a82f5958e48310477ba9c63d832e3095
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1753059
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686670}