GitLab

When enabled, the holding space tray will have an icon representative of
the holding space model contents.

As holding space items are added to the model, the icon is updated to
include a new visual representation of the added item. As holding space
items are removed from the model, stale representations are removed.

The visual representations for items are rendered using ui::Layers added
to the holding space tray icon which only exist when visible in the
viewport. This is to limit the number of layers if the user happens to
have a large number of holding space items.

Note that this CL just roughs things in. Still TODO:
- Animate bounds changes.
- Animate icon visibility changes.
- Render ImakeSkia's for items to their layers.
- Handle side shelf.

Changes are guarded by a new FeatureParam and disabled by default.

Screenshot: https://screenshot.googleplex.com/6g64LPbEUNc8o8U

Bug: 1142572
Change-Id: I3eada919a6d28ccfb208d1b15b8990d4762f3dd8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2499882Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#821778}