Fix memory leaks in content settings introduced by linked_ptr removal.

Bug: 556939, 914453
Change-Id: Ibd8757967a051507e0fc2a9ae93ac789e51b005b
Reviewed-on: https://chromium-review.googlesource.com/c/1425708
Reviewed-by: Jun Mukai <mukai@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Marek Haranczyk <mharanczyk@opera.com>
Cr-Commit-Position: refs/heads/master@{#625144}