Remove linked_ptr.

BUG=556939
TBR=rogerta@chromium.org
TBR=bbudge@chromium.org

Change-Id: I11091ef719405e2a716b1e75b9df86018aba26e2
Reviewed-on: https://chromium-review.googlesource.com/c/1381517
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#618695}
6 files changed