Remove linked_ptr from the extensions trackers.

BUG=582001

Change-Id: I4141f3fe268d34e5d078740b5ef12b4f02b5efb1
Reviewed-on: https://chromium-review.googlesource.com/c/1364550
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614633}
6 files changed