The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7cd1d105224a6375bc27689ce7d7968f0fe07919

commit 7cd1d105224a6375bc27689ce7d7968f0fe07919
Author: Naoki Fukino <fukino@chromium.org>
Date: Tue Jul 17 13:21:26 2018

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}
[modify] https://crrev.com/7cd1d105224a6375bc27689ce7d7968f0fe07919/chrome/browser/about_flags.cc
[modify] https://crrev.com/7cd1d105224a6375bc27689ce7d7968f0fe07919/chrome/browser/chromeos/file_manager/volume_manager.cc
[modify] https://crrev.com/7cd1d105224a6375bc27689ce7d7968f0fe07919/chromeos/chromeos_switches.cc
[modify] https://crrev.com/7cd1d105224a6375bc27689ce7d7968f0fe07919/chromeos/chromeos_switches.h
[modify] https://crrev.com/7cd1d105224a6375bc27689ce7d7968f0fe07919/tools/metrics/histograms/enums.xml