Finishing a chunk of data during streaming compilation caused background
tasks to be restarted unconditionally. However, restarting background
tasks is not possible after compilation has already finished. With this
CL we do not allow anymore to restart background tasks after they have
been finished.

R=clemensh@chromium.org
CC=mtrofin@chromium.org

Change-Id: I4c0a9761fb627f04b254f72e05873e29e7647eb0
Reviewed-on: https://chromium-review.googlesource.com/827008
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50115}