Remove linked_ptr from sync_sessions.

BUG=556939

Change-Id: I6c0b8e066b315e0b80160c3b4b5d4912be8bf62b
Reviewed-on: https://chromium-review.googlesource.com/c/1378580
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617153}
2 files changed