ErrorThrower is not concurrency safe, thus we should not use it from
background tasks. Instead, allocate an ErrorThrower whenever we
actually want (or might) throw.
Pass the ErrorThrower from step 4 to step 5 explicitly.

R=ahaas@chromium.org, mtrofin@chromium.org

Change-Id: Ifb6b16cab7939ec9c81e4f2db59ee42d5ddd7f85
Reviewed-on: https://chromium-review.googlesource.com/489501
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45015}