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Previously the recent files container managed both screen captures and
downloads type holding space item views. Now, separate view classes
are added to manage each respective section. As a result, these
sections will now animate independently of one another.

There is opportunity for a lot of additional refactoring that will be
done in a follow up CL. The existing pinned files section and new
screen captures and downloads sections can inherit from a common view
class. The existing pinned files container and recent files container
can also inherit from a common view class.

Bug: 1154998
Change-Id: I27af3c70859a8cd3da91b3ea7b4668910ebe7ca8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577938
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834490}