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Adds a way to update holding space icon preview layer without an
animation, and utilizes it when the icon previews are first being shown
(i.e. when the list of previews is updated from an empty list). In this
case, tray icon will have it's own visibility animation, so there's no
need for running a separate bounce in animation within the tray icon.

Also, updates how tray icon updates are scheduled - instead of updating
the tray visibility synchronously, and then scheduling a previews
update, the logic is changed to update previews icon synchronously, too.
(the throttling became less important now that additions of multiple
items to the holding space are done in a batch, instead of one-by-one).

BUG=1166014

Change-Id: I3211672072262e9fff75b04a3316c6049099c749
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2627169
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#843358}