The outer_zone_ is only used to determine if the graph may be
unverifiable.

R=bmeurer@chromium.org

Change-Id: Idad2bbb0d2a4ba9006c852276651e6780c1128c5
Reviewed-on: https://chromium-review.googlesource.com/566821Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46765}