After the first decoder error, the streaming processor should not be
called again. To enforce this, reset the {processor_} field. This also
makes the {ok_} field redundant.
Note that this refactoring is also necessary for a future CL which
makes the {StreamingProcessor} keep the {AsyncCompileJob} alive. By
resetting the processor, we also remove that link.

R=ahaas@chromium.org

Bug: v8:7921
Change-Id: I42f5ed26a8f26c3dc8db5676557a0d82021e132e
Reviewed-on: https://chromium-review.googlesource.com/c/1329179
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57435}