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The original GetSlotIndexForTabModelIndex served two purposes: getting
the slot for an existing tab, and getting the insertion index for a
new slot. This splits it into separate methods with common logic
remaining in a shared method.

The logic had a bug when requesting the first tab of a group while
tabs just before it are animating closed. This bug is fixed.

To help debug the associated crash, CHECKs are added.

Bug: 1138748
Change-Id: I6e0d797bb70e9aa1da813dbf1fecf228290e55b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2515363
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Reviewed-by: Taylor Bergquist <tbergquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824672}