Reason for revert:
The change from rtc::linked_ptr to std::unique_ptr broke a downstream project.
Original issue's description:
> Delete rtc::linked_ptr. Only use, in statstypes.h, replaced with std::unique_ptr.
>
> BUG=webrtc:6424
>
> Committed: https://crrev.com/36f74e55792cae19db8b222c29aa38d6e0eb1225
> Cr-Commit-Position: refs/heads/master@{#15588}
TBR=solenberg@webrtc.org,pthatcher@webrtc.org,hta@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:6424
Review-Url: https://codereview.webrtc.org/2576673002
Cr-Commit-Position: refs/heads/master@{#15589}