Guard crostini SharePath behind crostini-files flag

Bug: 878324
Change-Id: I6b65b1439aa984312b6e11e39c7b8ffe6398359f
Reviewed-on: https://chromium-review.googlesource.com/1243869
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594223}
3 files changed