The existing access to the signatures is plain wrong. This CL fixes
this.
Note that cross-instance indirect calls are only enabled since a few
days (https://crrev.com/c/778159), which is why this bug was not
detected before.

R=titzer@chromium.org

Bug: chromium:787910
Change-Id: Iaac4d1d85840c921eb8554c5094933ec8d987802
Reviewed-on: https://chromium-review.googlesource.com/787312Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49607}