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This is a reland of 26c8b12c

BUG=751309

Original change's description:
> Implement wait-for-logs functionality in user_session binary
> 
> This ports the wait_for_logs method from the Python script, and forwards
> the pipe write fd to the Python script.
> 
> Change-Id: Ief54da136894cd13e0761b9755aa3c7d51fe17f8
> Reviewed-on: https://chromium-review.googlesource.com/594807
> Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
> Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
> Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#491058}

Change-Id: I9daf4957636a99b5179975cf1db33ba101bc179d
Reviewed-on: https://chromium-review.googlesource.com/597087
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491179}