This is needed for proper deserialization of code and has the nice
side effect of fixing the nasty race condition that led us to
introducing a lock on the signature map.

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

Bug: 
Change-Id: I6a018344ad8b58b088b20756d3b00ae08232bbb9
Reviewed-on: https://chromium-review.googlesource.com/718937
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48589}