The barrier assumed that the number of tasks is fixed. However, we
cannot rely on that because other tasks might take up threads. In the
ein thend this would result in the Scavenge task being (rightfully)
cancelled.

The barrier now assumes no tasks in the beginning and relies on the fact
that reaching the barrier means that no global work is left. Tasks that
lag behing will just observe the barrier being in its end state.

Bug: chromium:738865
Change-Id: I4d47e8ec4b9cf7c615b3d9585e4a6bb9d271d409
Reviewed-on: https://chromium-review.googlesource.com/582947Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46842}