ErrorThrower::Reify() should only be called if an error is actually set.
This CL introduces a Reset() method to replace the obsolete (now
disallowed) usages.

R=mtrofin@chromium.org
BUG=chromium:717056

Change-Id: I41b989a9c7b33591ee26ec6d43540a38289ab54f
Reviewed-on: https://chromium-review.googlesource.com/493506Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45039}