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This CL customizes the deletion animation for tab grid cells via the
standard UICollectionViewLayout APIs.

Animation is done by adjusting the final layout attributes of the cell
in -finalLayoutAttributesForDisappearingItemAtIndexPath:. Because cells
may disappear for non-deleting reasons, the layout also needs to track
which cells are being deleted during each update cycle.

Bug: 804553
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I8566a5aff832ac8844d2d89639599b1b2372a27d
Reviewed-on: https://chromium-review.googlesource.com/964223Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543389}