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This change allows GuestOsApps to be visible in the shelf/launcher, and
associates its windows with those items.

The behaviour is only partially completed, the current items will be
finished in a forthcoming CL:
 - Support launching the apps from the launcher/pinned shelf items
 - Have context-menus for our shelf/launcher items.

Bug: b/162562622
Change-Id: I7456fb8173cf1964663709fc015ba9cc9b79df12
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454952
Commit-Queue: Nic Hollingum <hollingum@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Daniel Ng <danielng@google.com>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816059}