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If a tab wants to move to model index to the left of the leftmost tab
in a group, where it should end up in TabStripLayoutHelper::slots_ is
ambiguous. If it is in the same group as the grouped tab, it should go
to the right of the header so the group remains contiguous. But if not,
it should go to the left. To disambiguate this case,
GetSlotIndexForTabModelIndex now takes the group of the tab at
|model_index| as a parameter. It cannot look up the Tab* in
TabStrip::tabs_ in the general case, since it might be called in the
middle of processing a MoveTab or RemoveTab, where the model indices
have already been updated in that data structure.

Bug: 989964
Change-Id: I2f35218d01c8398e6e024096bd35244ca67ca330
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764181
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690053}