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HoldingSpaceItemViews fade in and translate up into place. When being
removed, they fade out and also translate up.

Demo: http://shortn/_UrRIQF1CXm

Still TODO:
- Animate container grow/shrink
- Animate pinned section empty label
- Animate section labels (when sections are going away)
- Animate screen captures and downloads sections independently of each
  other so that adding/removing items from one section does not impact
  the other unnecessarily.

Bug: 1154998
Change-Id: Ib28c35b79b20e265059c496c2c7c8ceb23defe6c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575322
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: David Black <dmblack@google.com>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834301}