First checkin of Crostini app model builder and related classes.

This is placeholder only and the button doesn't yet connect to any action.

Bug: 813699
Change-Id: Idc84916be75ee25ac1f570a5917af8363f6f5655
Reviewed-on: https://chromium-review.googlesource.com/925921
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538296}
20 files changed