Extract mount state tracking into separate class

BUG=chromium:916006

Change-Id: I800c77b39c7e1511a50b9f793355babf7b6f892c
Reviewed-on: https://chromium-review.googlesource.com/c/1426208
Commit-Queue: Sergei Datsenko <dats@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631120}
11 files changed