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It uses slide animation to be consistent with how other tray items -
mainly the unified system tray - animate their size. On each animation
step, it updates the tray icon preferred size, which updates the shelf
layout.

Adds a previews container view to HoldingSpaceTrayIcon, to make it
easier to compose the transform changes on the previews during resize.
A preview movement has two components:
*   shift animation to handle preview index change within the icon
*   offset due to icon size change. The icon is expected to grow
    towards the start of the icon, which is opposite of how bounds grow
    by default. As the size changes, the items have to shift so their
    position remains constant relative to the end of the icon.
The latter is handled by updating transform on the container view
layer as the resize animation progresses.

Bug: 1142572
Change-Id: I411844c9a9c8fc4a270feaa94530f34be7bae088
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601757
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#839201}