The CompileStep is only invoked via the {AsyncCompileJob} that owns it,
so we can just pass a pointer to the AsyncCompileJob instead of storing
it in the step itself.

R=ahaas@chromium.org

Bug: v8:8238, v8:7921
Change-Id: I92eda222ace2d5fef5af7663175c62fa7601810c
Reviewed-on: https://chromium-review.googlesource.com/c/1319759
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57291}