Revert of Delete rtc::linked_ptr. Only use, in statstypes.h, replaced bu std::unique_ptr. (patchset #1 id:1 of https://codereview.webrtc.org/2581663002/ )

Reason for revert:
This change broke Chrome too. It's stats processing wants to make a copy of webrtc's stats mapping, which is no longer possible with std::unique_ptr.

Original issue's description:
> Reland of Delete rtc::linked_ptr. Only use, in statstypes.h, replaced bu std::unique_ptr. (patchset #1 id:1 of https://codereview.webrtc.org/2576673002/ )
>
> Reason for revert:
> Downstream project fixed to not make copies while iterating over the stats mapping.
>
> Original issue's description:
> > Revert of Delete rtc::linked_ptr. Only use, in statstypes.h, replaced bu std::unique_ptr. (patchset #1 id:1 of https://codereview.webrtc.org/2567143003/ )
> >
> > 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
> >
> > Committed: https://crrev.com/8afbc8cba65d99bb7a0feece8fb3055b144106b1
> > Cr-Commit-Position: refs/heads/master@{#15589}
>
> TBR=solenberg@webrtc.org,pthatcher@webrtc.org,hta@webrtc.org
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=webrtc:6424
>
> Committed: https://crrev.com/06035cf53abad80b0525f286a3b81e388cc7ee00
> Cr-Commit-Position: refs/heads/master@{#15627}

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/2579753002
Cr-Original-Commit-Position: refs/heads/master@{#15629}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6a58f33450a46370039cd82537612040a91165b3
4 files changed