This add scope for
- concurrent array buffer free task,
- concurrent store buffer processing task,
- concurrent unmapper task

Bug: chromium:758183
Change-Id: I1be3185a9ec44033982f6a0bb05d2e9b02074f85
Reviewed-on: https://chromium-review.googlesource.com/811646Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49922}