This is also needed for the standard Chrome restart case.

Must also provide APIs for:

3/ Is crostini enabled, container exists.  This is used by FilesApp to determine whether to show the 'Linux apps' root at all.

4/ Possible API required to stop container when FilesApp starts it.  How does crostini currently stop?  Could each user of the container register as an observer, then container stops when no observers left? 

4/ There will be two ways to stop the container: 

a) shut down the chromebook
b) use a settings page (still being implemented).

Once 3) is available, files app should use the new CrostiniManager::RestartCrostini call before trying to list files.

APIs will be:

1/ IsCrosTerminaInstalled() - used by FilesApp to determine whether to show 'Linux apps' root.
2/ RestartCrostini() - ensures crostini is started.

http://crosreview.com/1013804

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/55e6170f799adfd4c2e9881bb81f4231e139ae14

commit 55e6170f799adfd4c2e9881bb81f4231e139ae14
Author: Nicholas Verne <nverne@chromium.org>
Date: Wed Apr 18 06:20:05 2018

Crostini restarting flow.

Once crostini is already installed, it needs to be restarted after
Chrome shuts down. These restart steps are all very fast in the case that
the default vm and container are already running. If not, there can be
a several second delay. Launching crostini apps always goes through this
flow and testing shows the impact is not user-noticeable when the
vm and container are running.

A future CL will add an observer to the
CrostiniRestarter class for this case.

Bug:  832509 
Change-Id: I359e5a25c30664fad6425c218853dda204c605d9
Reviewed-on: https://chromium-review.googlesource.com/1013804
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Timothy Loh <timloh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551593}
[modify] https://crrev.com/55e6170f799adfd4c2e9881bb81f4231e139ae14/chrome/browser/chromeos/crostini/crostini_manager.cc
[modify] https://crrev.com/55e6170f799adfd4c2e9881bb81f4231e139ae14/chrome/browser/chromeos/crostini/crostini_manager.h
[modify] https://crrev.com/55e6170f799adfd4c2e9881bb81f4231e139ae14/chrome/browser/ui/app_list/crostini/crostini_app_item.cc
[modify] https://crrev.com/55e6170f799adfd4c2e9881bb81f4231e139ae14/chrome/browser/ui/app_list/crostini/crostini_installer_view.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c683085f814a04c6eccd2e9a360b99d770960481

commit c683085f814a04c6eccd2e9a360b99d770960481
Author: Nicholas Verne <nverne@chromium.org>
Date: Mon Apr 23 08:24:02 2018

Refactors CrostiniInstallerView to use the new Restart flow.

A new RestartObserver interface is added to CrostiniManager for use
by the Restart flow. CrostiniInstallerView adds istelf as an observer and
much duplicated code is now deleted.

Bug:  832509 
Change-Id: If10451fa7efff228f6df38821270558df68ec367
Reviewed-on: https://chromium-review.googlesource.com/1016185
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Ben Wells <benwells@chromium.org>
Commit-Queue: Nicholas Verne <nverne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552651}
[modify] https://crrev.com/c683085f814a04c6eccd2e9a360b99d770960481/chrome/browser/chromeos/crostini/crostini_manager.cc
[modify] https://crrev.com/c683085f814a04c6eccd2e9a360b99d770960481/chrome/browser/chromeos/crostini/crostini_manager.h
[modify] https://crrev.com/c683085f814a04c6eccd2e9a360b99d770960481/chrome/browser/chromeos/crostini/crostini_manager_unittest.cc
[modify] https://crrev.com/c683085f814a04c6eccd2e9a360b99d770960481/chrome/browser/ui/app_list/crostini/crostini_installer_view.cc
[modify] https://crrev.com/c683085f814a04c6eccd2e9a360b99d770960481/chrome/browser/ui/app_list/crostini/crostini_installer_view.h
[modify] https://crrev.com/c683085f814a04c6eccd2e9a360b99d770960481/chromeos/dbus/fake_concierge_client.cc