{AsyncCompileJob::FinishCompile} assumes that a context is set on the
isolate when it is called. This was not the case when it was called at
the end of streaming compilation.

R=mstarzinger@chromium.org

Bug: chromium:874525
Change-Id: Ia31cd9eb40598918262ff14606796027a6c80bc0
Reviewed-on: https://chromium-review.googlesource.com/1177392
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55161}