The lifetime of the AsyncCompileJob does not depend on the lifetime of
the stream which feeds data into it. Multiple checks guarantee that the
AsyncCompileJob still exists when the stream wants to call it. With
this CL we add an additional level of defense to make sure that
streaming does not continue after the AsyncCompileJob got destructed.

It is not clear if this CL fixes the bug referenced below. However, the
crashes there could be caused when streaming accesses the
AsyncCompileJob after it got destructed already. I was not able though
to find a scenario where this is possible.

R=clemensh@chromium.org

Bug: chromium:888170
Change-Id: Id5c6cc34842735a3adaf3e09c57cbe923cfc2630
Reviewed-on: https://chromium-review.googlesource.com/1241961
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56213}