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Implementing an installer for borealis, based on the install flow of
Plugin VM and Crostini. Currently there is no way of accessing the
installer (during testing I attached it to a dummy app entry I created).
Whilst the install process is quite lean right now, I left in some
structures, inspired by the other installers, that will be useful when
the installation flow gets more complex and/or when tests are added.
I haven't added any tests since things may be scrapped when proper UX
is decided, the installer is only being used internally and it exists
in a vacuum.

For more context into the installer, and Borealis, please see the
attached bug.

Tests: Tested manually on my machine.
Bug: b:161650651
Change-Id: I552c39f2ff4e0da86bc0f113bef6c4129e671a51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2338275
Commit-Queue: Daniel Ng <danielng@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Nic Hollingum <hollingum@google.com>
Reviewed-by: Dana Fried <dfried@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797571}