Remove linked_ptr from extensions message rules.

BUG=556939

Change-Id: If65afd6e3c66bfda4e94a73624f473462de714d5
Reviewed-on: https://chromium-review.googlesource.com/c/1374161
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616137}
3 files changed