See referenced bug: Async compilation can deadlock if a background task
queues the last compilation unit to be finished while the finisher
is already exiting because there was no more work.
This CL fixes this by making the finisher task check for new work after
setting the finisher_is_running_ flag to false.

R=ahaas@chromium.org
CC=kimanh@google.com

Bug: chromium:824681
Change-Id: If1f5700a9fdd5d150b36e37a5d14b692c2b0f3fb
Reviewed-on: https://chromium-review.googlesource.com/975301
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52139}