This enables concurrent marking for x86 and x64 on GYP builds.

Bug: chromium:694255
Change-Id: I371b38e72ce0e8f7ad5b0eed4e29b223b9ed1cf4
Reviewed-on: https://chromium-review.googlesource.com/628836
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47551}