It was stored in a shared_ptr so far, which makes it more difficult to
reason about life times. Since there is always exactly one owner of the
AsyncCompileJob, a unique_ptr actually suffices.

R=ahaas@chromium.org

Change-Id: If94c9091889ad05325c559a97e9a9ffeee8d450c
Reviewed-on: https://chromium-review.googlesource.com/968604Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52052}