The function shouldn't try to start black allocation if it is already
started.

Bug: chromium:694255
TBR: mlippautz@chromium.org
Change-Id: I77b5346f6ac2ec5947ca4351a8abe33865729fda
Reviewed-on: https://chromium-review.googlesource.com/612385Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47334}