In a certain scenario streaming compilation got stuck and did never
finish. This CL fixes this issue.

Scenario:
* Streaming compilation starts
* The compilation tasks execute all compiation units in the working
  queue and set the finished_ flag to true.
* New data arrives over streaming
* The compilation tasks compile so fast that the executed_units_ queue
  gets full. The compilation tasks stop executing and wait for the
  finisher task to restart them.
* The finisher task does not restart the compilation tasks because the
  finished_ flag is set.

With this CL I remove the finished flag and instead look at the size
of the working queue directly.

In addition I added a test which does not actually reproduce this
scenario but seems good to have anyways.

R=mtrofin@chromium.org

Change-Id: I44560c43e51be13c4461208368e21137b115656c
Reviewed-on: https://chromium-review.googlesource.com/824523Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50090}