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This adds a fake local ARC volume for testing. Initial integration
tests added check that
1) the "Toggle Play Folders" button work in hiding and showing the
   non-default Android files and folders.
2) the "Toggle Play Folders" button is shown inside Play Files but
   hidden from the "Recents" pane.

Bug: 877371
Test: Tested on Chromium build.
Change-Id: Ic35de5ebb06f901ad229c9b35f1917ac9353a459
Reviewed-on: https://chromium-review.googlesource.com/1201637
Commit-Queue: Alex Lau <alexlau@chromium.org>
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589847}