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This is a reland of ac0ccdc1 with
FileManagerBrowserTestBase::drive_volume_ null-initialized.

Original change's description:
> Add a FakeDriveFs and change FileManagerBrowserTestBase to use it.
>
> Bug: 848126
> Change-Id: Icb7b5edf28aa61e133aff0f0bf639f06b32adcfa
> Reviewed-on: https://chromium-review.googlesource.com/1088336
> Commit-Queue: Sam McNally <sammc@chromium.org>
> Reviewed-by: Noel Gordon <noel@chromium.org>
> Reviewed-by: Sasha Morrissey <sashab@chromium.org>
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#566333}

Bug: 848126
Change-Id: I583fd6ebe92e5f191de93a3b3167a1c50e595a95
Tbr: noel@chromium.org, sashab@chromium.org, xiyuan@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1098395Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566691}