This patch ensures that the concurrent marking tasks do not
use more than the half of the available background threads.

Bug: chromium:694255
Change-Id: I67d6eb3e717945f777d0711bd094630573c78661
Reviewed-on: https://chromium-review.googlesource.com/678636
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48130}