and use it for parallel GC as it blocks the main thread.

On Chromium this will result in a TaskPriority::USER_BLOCKING task,
allowing TaskScheduler to prioritize them above all other async work.

R=ahaas@chromium.org

Bug: chromium:817421
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I8c782eb045035ce2899214528deae5a45f0e6600
Reviewed-on: https://chromium-review.googlesource.com/941946
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51650}