We need to modify that module, e.g. to add signatures to the signature
maps. Hence it cannot be const.

R=titzer@chromium.org
CC=mtrofin@chromium.org

Change-Id: I261af5b4233a0b2ec8031a9cbe0cf9f826316ae0
Reviewed-on: https://chromium-review.googlesource.com/600428Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47138}