We need the job_'s context to reify errors, for example.
Some tasks already managed their context, so centralized
that logic in a SyncCompileTask all sync tasks derive from.

Bug: 
Change-Id: I90f462476b47bb8d5777db93d8dfb5c72fbd885b
Reviewed-on: https://chromium-review.googlesource.com/495392
Commit-Queue: Mircea Trofin <mtrofin@google.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45112}