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Addresses two issues:
1. When shelf alignment/config changed any previews that were in the
   process of being removed were appropriately destroyed. However, we
   failed to call `OnOldItemsRemoved()` after so any new/existing items
   weren't animated/shifted into position. The result was missing and
   misplaced previews.
2. The translation calculation in `OnShelfConfigChanged()` was slightly
   off. Fixed with index based calculation.

Bug: 1166804
Change-Id: Ifda6a6a4fa527bfc6753840a7257fc92c9446a20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631293Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#843890}