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If drivefs needs to restart itself it will inform the browser and unmount,
and the browser will try to mount again after specified time.

BUG=chromium:845390

Change-Id: Idb8e7767ef53469c7c8bc96ff0b0bf8c028a68b0
Reviewed-on: https://chromium-review.googlesource.com/1134712Reviewed-by: Sam McNally <sammc@chromium.org>
Commit-Queue: Sergei Datsenko <dats@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575200}