FilesApp crostini integration test

* Test verifies fake root is shown, then mounts container
  and verifies that correct volume and contents are displayed.
  Unmounts volume, and verifies that fake root is shown again.
* Sets prefs / features for crostini and registers mount callback
  with FakeCrosDisksClient to simulate sshfs mounting.

Bug: 845075
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ice3f96430ef063a9942fdd2c40e99b30b35b0c50
Reviewed-on: https://chromium-review.googlesource.com/1094574
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571344}