GitLab

It uses a blocking PathExists call under the hood, so cannot be called
sync on the UI thread.

The IsRegistered check now runs when MaybeUpgradeCrostini runs, which is
at user session startup time

Bug: 870117
Change-Id: I000887e2e76a0cbebfe7c12a37fe1e7d7911e22d
Reviewed-on: https://chromium-review.googlesource.com/1159936Reviewed-by: Timothy Loh <timloh@chromium.org>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580091}