Give feedback to user if crostini install is canceled

This CL changes the crostini installer to show a message and disable
the cancel button if the cancel button is pressed. This gives the user
an indication that their request has been acknowledged.

Bug: 930583
Change-Id: Icf23af47bb1676ad286adf1f43a316c18c7ae2f1
Reviewed-on: https://chromium-review.googlesource.com/c/1477562
Reviewed-by: Timothy Loh <timloh@chromium.org>
Commit-Queue: Fergus Dall <sidereal@google.com>
Cr-Commit-Position: refs/heads/master@{#633633}
3 files changed