The code was already there, but there was a bug in it: Because of the
missing reference, we were only updating a *copy* of the signature map,
hence the update had no effect.
This intentially is a minimal CL, in order to allow for easy
backmerging.
More mitigations and tests are coming in a separate CL.

R=titzer@chromium.org

Change-Id: Ifb462093f4b8f4d5380b6774636537c67c2b676c
Reviewed-on: https://chromium-review.googlesource.com/570278Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46664}