Add DocumentsProvider roots as Volumes in VolumeManager.

With this change, Chrome retrieves available DocumentsProvider roots
from ARC container and show them in Files app if a flag is enabled.

DocumentsProviderRootManager class manages the root list.
If its SetEnabled() is called or it is notified any updates of root list,
it retrieves all available roots from ARC container and notifies
newly-added roots and deleted roots to its observer (i.e. VolumeManager).
VolumeManager adds/removes those roots as its volumes.

TBR=ellyjones@chromium.org

Bug: 905974
Test: Run unit_tests
Change-Id: I036999e4cff3d298928edf1c5f45334fcabe5ca2
Reviewed-on: https://chromium-review.googlesource.com/c/1414252
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Reviewed-by: Noel Gordon <noel@chromium.org>
Reviewed-by: Shuhei Takahashi <nya@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624745}
23 files changed