Files app: Show "Play files" root by default.

Bug: 864449
Test: Manually tested both Enable/Disable options.
Change-Id: I57a768421e3a86e1bae63c72f1467911ddb6096f
Reviewed-on: https://chromium-review.googlesource.com/1140110
Reviewed-by: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575623}
5 files changed