Fix task num calculations in run_lua_on_workers
Fix task num calculations in run_lua_on_workers

BUG=skia:6178

Change-Id: I86da970ef7378386d95e31300f94ba4deee6d7a1
Reviewed-on: https://skia-review.googlesource.com/7820
Reviewed-by: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>
Commit-Queue: Ravi Mistry <9fa2e7438b8cb730f96b74865492597170561628@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot