If a job was already enqueued, EnqueueAndStep would still step the job one
more step. However, since it didn't take the job out of the
pending_background_jobs pool, the job could get picked up by a background
thread which would try to step it, but it the job is now at a step which
can't be run on the background.

BUG=v8:5203,chromium:685515

Change-Id: I2cee2a33625ba455aca49a8037601be9ff8bb73f
Reviewed-on: https://chromium-review.googlesource.com/441084
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43121}