This also starts black allocation earlier if concurrent marking compile
time flag is on.

Bug: chromium:694255
Change-Id: I73c02676e5149fae10e5f9301ad585926e223a1d
Reviewed-on: https://chromium-review.googlesource.com/618893Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47412}