rearrange wg use

There's no need to tick wg up and down when running reruns, and as
written it's possible for the overall fm_driver program to exit before
one last call to endStep() has happened.  Simply calling wg.Done() once
per item dequeued outside worker() fixes both.

Change-Id: I0fb0acc5a3f2c624dfc14f875fa094db6dd40838
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/366137
Reviewed-by: Mike Klein <mtklein@google.com>
1 file changed