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These serve as common model classes that can back either
UICollectionView or UITableView.  CollectionViewModel is updated to be a
subclass of ListModel with added restrictions on permitted item types
but no functional changes.  CollectionViewController and its subclasses
are largely unchanged.

BUG=805136

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ibcfbab5bae62c5c8a37f764508294421d3d1c307
Reviewed-on: https://chromium-review.googlesource.com/916365
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536708}