GitLab

Adds a feature flag and general code structure that will be used for the
feature:
*   chrome/browser/ui/ash/holding_space - browser context keyed service
    that keeps track of per-user holding space items, and that will be
    used as an entry point to add items to the holding space from
    Chrome.
*   ash/public/cpp/holding_space - general holding space data model,
    that contains information about items that have been added to the
    holding space, and that propagates model changes between chrome
    and ash
*   ash/holding_space - the UI implementation for the holding space.

The logic is pretty bare, mostly focused on propagating model state
updates to ash. The UI implementation, and entry points to add items
will be done in coming weeks.

BUG=1106991

Change-Id: Id0a70929aecfe4164b613a39db42b9412368abbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2310902Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791013}