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This CL provides expand and contract animations for views that are being
shown or hidden. It should cover the needs of the identity chip and
butter.

Still to do:
 - Move animating_layout_manager to ui/views/layout
 - Rename flex_layout_types_internal to something more appropriate
 - Implement slide and fade animations
 - Handle multiple views being added/removed at once more cleanly
 - Allow per-child-view transition rules instead of a per-layout default

Bug: 1000131
Change-Id: Ia73475e9eda7e72c118a02160b3fc7234474ee83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1782040Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Auto-Submit: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693826}